Rebel Serbs in Bosnia Harass U.N., Shell Bihac Pocket : Balkans: Second day of artillery attacks kills two in northwest enclave. Peacekeepers are duped in capital.

SARAJEVO, Bosnia-Herzegovina —  As if to prove that a wobbly cease-fire will not necessarily mean lasting peace, Bosnian Serb rebels Sunday heaped fresh humiliation on the U.N. Protection Force and pressed a deadly attack on the “safe area” of Bihac.

A young woman and a teen-age girl were killed in a second day of heavy artillery shelling of the northwestern enclave--a 19-year-old cut down by a mortar that struck a schoolhouse and a 15-year-old killed while sitting at home with her mother.

At Sarajevo’s airport, gunmen loyal to Bosnian Serb leader Radovan Karadzic tricked French peacekeepers for the second straight day into opening a front-line crossing for their traffic, then forced the U.N. soldiers to close it once it was time for Bosnian government loyalists to pass.

The rebels’ brash military chieftain, Gen. Ratko Mladic, also informed the U.N. mission that it should not try to move troops or supplies across their territory to U.N.-protected enclaves, claiming that winter weather makes the roads unsafe for travel.

Fighting across Bosnia-Herzegovina has largely died down since a cease-fire was arranged by former U.S. President Jimmy Carter in mid-December. The cease-fire was expanded to a four-month truce on New Year’s Eve.

Mortar blasts, heavy machine-gun fire and persistent sniping have all but ended in this ravaged capital, and only sporadic clashes have been reported along the 1,000-mile front line snaking through Bosnia.

But attacks by Bosnian Serbs and their Croatian Serb allies in the Bihac pocket have escalated in recent days, exposing the fragility of an agreement that many here fear was signed only to provide both sides with a respite during the snow season.

Seven civilians were killed in Bihac over the weekend, and the snubs to the U.N. mission over the Sarajevo airport crossings and access to Serb-encircled enclaves demonstrated the rebels’ confidence that they continue to hold a commanding position despite the Carter cease-fire.

Two routes for civilian and humanitarian traffic were to have opened across Sarajevo airport Saturday, according to a U.N.-brokered agreement. The crossings are intended to restore movement between the Serb-held communities lying east and west of the airport and between the captive city center and government-held land to the south.

The pact proposed to let Serbian traffic cross east and west from 8 to 10 a.m. and from 1 to 3 p.m., and for government transit north and south across the front-line airport from 10 a.m. to noon and from 3 to 5 p.m.

On Saturday, four carloads of Serbs crossed during the first shift, but when it came time for the 10 a.m. changeover, rebel gunmen threatened to shoot any Bosnian civilians using the route, prompting the French peacekeepers responsible for the airport to close it.

A Serbian liaison officer near the airport told the French early Sunday that the Serbian leadership had relented and that the routes would be allowed to operate.

Risley said several hundred Serbs used their crossing between 8 and 10 a.m. but that the rebels again vowed to shoot at government traffic.

The U.N. commander for Bosnia, British Lt. Gen. Michael Rose, traveled to the rebel stronghold of Pale, about 10 miles east of here, in an attempt to resolve the airport situation, the persistent fighting around Bihac and the provocative order from Mladic forbidding U.N. travel in Serb-held land.

But little headway was made, and Rose returned from his meeting with Karadzic with new conditions imposed by the Serbs that would limit Sarajevans’ use of the airport routes to a degree already deemed unacceptable by the government.