PARIS (AP) _ Western leaders defended their bombardment of Bosnian Serb positions today as a necessary response to the Serb slaughter of 37 people at a Sarajevo marketplace. Russia, the Serbs' traditional ally, called the NATO airstrikes cruel.

Troops from NATO and European countries launched a blitz against Serb positions in Bosnia after the Bosnian government threatened to pull out of peace talks unless Western countries responded to Monday's shelling.

``I believe it is something that had to be done,'' said President Clinton, who is vacationing in Jackson Hole, Wyo.

President Jacques Chirac of France said the bombardment ``marks our determination to make the security zone around Sarajevo respected.''

The attacks were a ``necessary and consistent reaction of the international community,'' said Foreign Minister Klaus Kinkel of Germany.

``Those who terrorize tens of thousands of innocent people with artillery attacks, sniping operations and the strangulation of the city of Sarajevo deserve a hard answer,'' he said.

In London, Prime Minister John Major said NATO and U.N. nations ``responded robustly and properly to Monday's ruthless attack by the Bosnian Serbs.''

And Foreign Affairs Minister Andre Ouellet of Canada said the strikes were needed to respond to the ``barbaric'' shelling.

``Attacks on safe areas cannot be tolerated and will not go unanswered,'' he said in a statement. ``These airstrikes demonstrate our resolve to respond to such outrages.''

Russia denounced the Serb attack and the NATO airstrikes in the same breath.

``We condemn both the first act and the bombing raids,'' President Boris Yeltsin said on Russian television. ``And we still oppose the use of force to solve the Yugoslavian crisis.''

His press secretary, Sergei Medvedev, spoke to the ITAR-Tass news agency of ``cruel acts of bombing and shelling of Serb positions by NATO.''

The Russians, who like the Serbs are Orthodox Slavs, have traditionally allied themselves with the Serbs.

In the first official reaction from the Bosnian Serb leadership, their self-styled foreign minister, Aleksa Buha, told the Bosnian Serb news agency SRNA: ``NATO stepped over the line of its involvement in the conflict.''

``Alleged Serb shelling of Sarajevo was not the real reason for this ridiculous act by the international community,'' he said, without elaborating.

Some countries worried that the attacks could endanger the peace process. A U.S. negotiating team, led by Assistant Secretary of State Richard Holbrooke, has been meeting in Paris with European and Bosnian officials, and flew to Serbia today. Holbrooke had no immediate comment on the attack.

The Ukrainian Foreign Ministry said the attack ``threatens the recently emerged positive trends regarding a peace settlement,'' the Interfax news agency reported.

Bosnia's president, however, said the attacks were a long-awaited sign ``that we are moving toward peace.''

``The world has finally done what it should have done a long, long time ago,'' Alija Izetbegovic told reporters outside Elysee Palace, where he met with Chirac. ``We see this not as the beginning of war, but the beginning of peace.''