Sarajevo Bombardment: U.N. Troops Duck, Watch : Balkans: Soldiers and postal workers in Bosnian capital have front-row seats for battle.

U.N. soldiers and Bosnian postal employees ducked for cover under the windows of darkened upper-floor offices as the clatter of sniper fire burst from high-rise tenements opposite.

Moments later, realizing that the shots were not directed at them, they smiled with relief and went back to watching the spectacular nighttime battle unfolding below.

The bombardment, described by U.N. officers as one of the heaviest so far in Sarajevo’s two-month siege by Serbian forces, erupted Friday and lasted well into Saturday.

Bright orange and yellow flashes illuminated the skyline and blast waves rocked the plate-glass windows of the five-story post and telecommunications building, now serving as U.N. headquarters. Red and white tracer bullets streaked past the windows. Others floated lazily skyward.

The fighting began just hours after the evacuation of about 800 soldiers and their families from the last remaining Yugoslav army barracks in central Sarajevo and an agreement to reopen the Serb-held international airport for desperately needed relief flights.

“This just goes to prove that whenever you hear an agreement is in the works, you better start looking for the nearest shelter,” said Bejaz Mameti, a postal mechanic.

French soldiers, dressed only in shorts and T-shirts, peered over windowsills and spotted the position of a multibarrel rocket launcher 500 yards away firing at Serb hillside positions.

Muzzle flashes could be seen on the crest of the hill to the east of the city, where Serb howitzers and mortars are positioned.

The United Nations scaled down its operations in the war-battered capital last month. Resentment against U.N. forces by local, irregular militias had been growing and the blue-helmeted troops had even been targeted.

Bullet holes in the headquarters’ bronze-tinted windows attested to past hits. The top three floors were evacuated after mortar rounds hit the roof three weeks ago.

Flames rose high from the roof of the nearby Rainbow Hotel, home to about 100 U.N. personnel and a dozen Bosnian maintenance workers.

“We all have to learn to live with the fighting. It has become a fact of life,” said Cpl. Johan Sillfors, a Swedish army cook. “I will be very surprised if it were suddenly to become peaceful.”