Three U.N. planes were struck by bullets Thursday in Sarajevo, forcing officials to suspend a vital humanitarian airlift restarted just two days ago.

Bullet holes were discovered on the planes after they returned safely to bases in Ancona, Italy, and the Croatian port of Split, said Maj. Dacre Holloway, a U.N. military spokesman.The planes were hit on the ground at the Sarajevo airport, said Peter Kessler, a U.N. aid spokesman. There was no immediate word on who was suspected of firing the shots.

The airlift, a lifeline for the besieged capital during most of the 28-month Bosnian war, was suspended more than three weeks ago after planes were hit by bullets believed to have been fired from Serb positions. It was restarted Tuesday.

"The situation in Sarajevo is extremely grave," Kessler said. "We have almost no food stocks in the city."

The shooting was another sign of mounting tensions around Sarajevo. Serbs have tried to tighten their siege of the city and Muslim-led government forces have gone on the offensive.

U.N. peacekeepers said as many as 3,000 Bosnian government troops have moved into a combat zone just north of Sarajevo.