4 Die as Clashes Erupt in Breakaway Croatia : Yugoslavia: In neighboring Slovenia, independence is celebrated with fireworks despite an air of tension.

ZAGREB, Croatia —  Ethnic violence flared in Croatia on Wednesday after the republic’s declaration of independence from Yugoslavia, killing at least four people and pitting Serbs against Croats in a conflict that many fear could escalate into civil war.

In neighboring Slovenia, tensions were high a day after the republic also announced its secession, but no violent confrontations were reported.

Army tanks took up positions near one border crossing into Italy, the Ljubljana airport was closed and military jets and helicopters swept the Slovenian capital in a show of force by federal authorities.

But officials who proclaimed Slovenia sovereign on Tuesday said they had regained control of all border facilities by the end of a day marked by verbal sparring and military posturing.

By nightfall, Slovenes felt confident enough to celebrate their newly declared independence with an emotional ceremony and fireworks that drew tens of thousands to Ljubljana’s Liberation Square.

The clashes in ethnically mixed regions of Croatia raised the republic’s death toll to at least 26 since early May and seemed to bear out Western governments’ fears that secession would unleash a new wave of hostility.

Two Croatian police officers and a Serbian gunman were killed when extremists attacked the police station in the town of Glina early Wednesday. A fourth fatality was reported in the town of Brsadin, near Croatia’s border with Serbia, when a Croatian militia reservist was shot to death during a gun battle.

The violence prompted federal troops and tank units to seal off Glina and Dvor na Uni, where another ethnic clash was reported.

A Western diplomat based in Zagreb said gunfire was heard in several towns in Slavonia and Banija, both regions of eastern Croatia that have large Serbian populations.

“The level of violence is not any greater than we’ve seen in the past few months,” the diplomat said. “It’s not clear yet if this is the beginning of a new phase.”

Croatia’s 600,000-strong Serbian minority strongly opposes secession from Yugoslavia, which would cut them off from Serbia. Militant Serbs in the region known as Krajina have declared themselves independent and have launched an armed uprising to break free of Croatian control.

Serbian rebels in Croatia plan to declare a territorial union with Serb-dominated regions of neighboring Bosnia-Herzegovina on Friday--a move that might be seen as another provocative attempt to redraw the internal borders of Yugoslavia.

Federal government action to protect the disintegrating federation appeared to be targeted at Slovenia, which unequivocally declared its independence from “the former Yugoslavia,” while Croatia seemed to suggest that further steps to full autonomy would have to be taken in the future.

In Belgrade, the federal government of Prime Minister Ante Markovic issued a tough statement after an overnight emergency session, ordering the Yugoslav army and Interior Ministry forces to seize control of Slovenia’s newly erected border crossings.

Slovenian Foreign Minister Dimitrij Rupel said there were “exchanges of words” between federal troops and republic border guards.

“So far, it’s a war of nerves,” Foreign Minister Rupel said. “I have no knowledge of any conflict.”

Early today, however, federal army tanks rolled into Slovenia’s main airport, the Associated Press reported. Defiant citizens had set up blockades but apparently did not try to fight the federal advance.

Slovenian officials said the federal government was trying to secure the field so military planes could land. A duty officer at the airport, Marjan Miklavci, said on Slovenian Radio that at least 10 tanks had entered the airport, just outside the Slovenian capital, Ljubljana.

The federal army’s apparent failure to act Wednesday on the order to take over Slovenian border crossings underscored Belgrade’s problems in enforcing unity.

Slovenia would consider any federal intervention as a violation of its rights to sovereignty and self-determination, Rupel, the republic’s foreign minister, had said Wednesday.

Slovenian border guards removed the Yugoslav flag from checkpoints along the Austrian and Italian borders and replaced “Welcome to Yugoslavia” signs with greetings from the new state.

Interior Minister Igor Bavcar said republic forces had regained control of all border posts by the end of the day.

Along the main highway between Ljubljana and the Croatian capital of Zagreb, Slovenian police stood a casual guard in the scorching sun while workers put the finishing touches on a hastily erected border checkpoint.

“Today you just don’t know. Anything could happen,” said Milan Podlesnik, a submachine-gun-toting policeman who took to stopping cars for the benefit of Western photographers.

While the federal government has sought to force unity among the six republics, it seemed to have little leverage in encouraging the army to get involved in a political conflict that could result in its own unraveling along ethnic seams.

The 180,000-man Yugoslav military, which is predominantly commanded by Serbian officers but has a multi-ethnic rank and file, was ordered by the federal government more than a month ago to disarm illegal militias and keep the peace in volatile areas of Croatia and the ethnically diverse republic of Bosnia-Herzegovina.

Those orders were never carried out, presumably because army officers fear mass desertions and a breakdown along ethnic lines.

Western governments have refused to recognize Slovenian or Croatian statehood, in part for fear that the secessions will set off further waves of ethnic unrest.

Secretary of State James A. Baker III described Yugoslavia as “a truly powder-keg situation.”

“The likelihood of more violence and bloodshed and disintegration is growing,” Baker said in a speech to the Gannett Foundation in Washington on Wednesday.

Baker, who failed last week in a last-minute attempt to persuade Slovenia and Croatia to abandon their secession plans, said that in Belgrade, “I found an air of unreality, an inability on the part of several republics’ leaders to understand the dangerous consequences of their actions.”

President Bush appealed to the battling republics to settle their differences through negotiation, saying, “We don’t need any more violence in the world . . . .”

But many Yugoslavs see the breakup as inevitable, despite the prospect of a damaging spiral of violence that could unleash a massive refugee wave or rekindle conflicts involving neighboring countries.

“If the Serbo-Croat conflict turns into a war, it would be the biggest European bloodshed since the Spanish Civil War,” Croatian security adviser Frans Visnar predicted in an article in the Zagreb weekly Globus. He said the ethnic clash could prove worse than the war that has raged in Lebanon for more than 15 years.

While formal recognition of Slovenian and Croatian sovereignty has been universally denied, some foreign governments sent representatives to Slovenia’s independence ceremonies on Wednesday night.

Austria, Norway, Thailand, the Central African Republic and the Republic of Tonga officially accepted the Slovenian invitation, Ljubljana authorities said.

The two secessionist republics have not ruled out reunion with the other four Yugoslav republics if Serbia will accommodate their demands for a loose alliance of independent states. Because Serbia is the largest republic and home to the federal capital, Belgrade, Serbs traditionally have dominated the centrally ruled federation.

Slovenia, with its ethnically homogeneous population, faces far less resistance to its independence than does Croatia.

In both cases, however, the federal government has deemed the actions illegal.

“The federal Interior Ministry will directly cooperate with the Defense Ministry to engage federal army units to secure state borders, at border crossings and in inhabited areas in border regions,” the government said in a statement.

“The installation of the so-called border crossings within the territory of Yugoslavia is forbidden. The border crossings that have already been set up must be removed,” the Tanjug news agency reported, quoting the government statement.

Yugoslavia’s collective presidency, which was charged with commanding the armed forces, collapsed in mid-May when Serbian delegates prevented the inauguration of Croatia’s Stipe Mesic as head of state.

Tanjug said the collective state presidency will meet today, and there was speculation that Yugoslav authorities might declare a state of emergency in a last-ditch effort to hold the nation together.

Times staff writer Norman Kempster in Washington contributed to this report.

The Problems of Nationhood

Independence declarations by Croatia and Slovenia have produced few immediately visible political changes. There are no separate passports or currency, and some federal troops remain. What problems face the two would-be nations?

RECOGNITION: Neither the United States nor Western European nations have recognized their independence. This is considered critical.

LAW: Slovenia annulled the validity of the Yugoslav constitution. Croatia was more vague. Legislation proposes transfer of customs and air traffic control to Slovenia. Slovenia hung its own flag from border posts, and it plans to take over federal army property by 1993.

RESISTANCE: Slovenia and Croatia each have about 10,000 men under arms. By contrast, the Yugoslav military has 180,000 members along with tanks and armor.

CURRENCY: Slovenia moved to establish a central bank to issue currency within eight months. Croatia also plans to issue its own currency.

GOVERNMENT: Slovenia is ruled by Demos, a coalition of six center-right parties that last year beat the reformed Communist Party in the first multi-party elections in 45 years. Its president is Milan Kucan. The Croatian Democratic Union won last year and Franjo Tudjman was named president. Tudjman, a former Communist general, is a popular champion of nationalism.

ECONOMY: Slovenia’s industries, including the Elan ski factory, are suffering under introduction of free-market reforms and unemployment is up. It produces 30% of Yugoslavia’s hard-currency exports. Croatia’s Adriatic coast accounts for about 80% of Yugoslavia’s $3-billion tourism business, but tourism has been halved by ethnic violence. Other industries are shipbuilding, petrochemicals, food processing and agriculture. Croatia accounts for about 25% of Yugoslavia’s output.