Rebels Machine-Gun Trams in Sarajevo : Balkans: One man is killed, six other people are wounded in attack. Motive is apparently vengeance.

Bosnian Serbs raked three Sarajevo trams with machine-gun fire Saturday, killing one man and seriously wounding six other people, including children, in an apparent revenge attack.

The shooting followed a warning by hard-line Bosnian Serb leader Momcilo Krajisnik that Serbs would avenge the killing of 20 Serbian soldiers and nurses in a commando raid by the Muslim-led Bosnian army.

Witnesses Saturday saw some victims hit several times during a 12-minute fusillade aimed at Sarajevo’s notorious “sniper alley.” It forced U.N. peacekeepers to deploy armored cars as a shield against the attackers.

Hospital doctors treated six badly wounded, included boys aged 14 and 16. Five other children were less seriously hurt.

It was the worst Serbian attack on Sarajevo’s civilian population since a bombardment last month that killed two people and wounded 18.

U.N. spokesmen said the firing came from Bosnian Serb army positions in Sarajevo’s Jewish cemetery.

Yasushi Akashi, the U.N. special envoy for the former Yugoslav federation, condemned “this flagrant and deliberate attack on civilians.”

Krajisnik had told the Bosnian Serb news agency SRNA on Friday night that “Serbs cannot be passive” after the killing of 20 soldiers and nurses the previous day. The victims were buried Saturday.

The United Nations described the attack on the Serbs, at a Bosnian Serb army command post outside Sarajevo, as a military operation by the Muslim-led Bosnian army, whereas the Serbian targets Saturday were civilians.

Serbian revenge attacks on Muslim civilians for military reverses have been characteristic of their tactics during 30 months of fighting in which 10,000 people have died in Sarajevo alone.

Also Saturday, the Office of the U.N. High Commissioner for Refugees halted aid flights to Sarajevo after two U.N. military planes were hit on the runway Friday.

The airport had reopened only Friday after being shut since Sept. 22 because of Serbian threats to shoot at aircraft in retaliation for a NATO air strike. A spokesman said the flights will resume today.

The Serbian blockade severely disrupted efforts to keep Sarajevo’s 380,000 people supplied with food.

U.N. official Kris Janowski said bakeries and warehouses in Sarajevo had stocks for only a few days.