PARIS -- Leaders from former Yugoslavia signed a Bosnian peace treaty Thursday, formally ending Europe's worst conflict since World War II and opening the way for thousands of NATO troops to move into the shattered country.


But the ink was barely dry when sporadic violence flared again in Bosnia, with explosions in districts of Sarajevo, government troops firing on a French UN helicopter and Croat forces clashing with pro-government Islamic fighters.


Serbian President Slobodan Milosevic, Croatian President Franjo Tudjman and Bosnian President Alija Izetbegovic signed the accord in Paris, before a glittering assembly of leaders from major powers including U.S. President Bill Clinton.


All of them pledged their support for the tough task of making the agreement a reality, urging the warring parties to put their enmity behind them after more than three years of war and promising military, financial and moral support.


"I feel like a man who is drinking a bitter but useful medicine," said Izetbegovic.


"Seize this chance and make it work," Clinton told the presidents of Bosnia, Croatia and Serbia. "You can do nothing to erase the past but you can do everything to build the future. Do not let your children down."


Between two and three million people have been displaced by the war and some 200,000 have been killed.


Recalling the horrors of children whose playgrounds were shelled, young girls raped, victims piled in mass graves, ethnic cleansing, floods of refugees and starvation, Clinton and others said those ac backing from Congress for providing 20,000 U.S. troops as the backbone of the NATO force. Preliminary deployments have been hampered by heavy snowfall.


Later, Clinton promised in a television interview that U.S. troops would not "cut and run," despite the risk of casualties and memories of the disastrous intervention in Somalia.


In Sarajevo, there was some celebratory gunfire to mark the end of the war and UN officials said they had won a pledge from the Bosnian army's most successful fighting unit -- the 5th Corps -- that it would not block the movement of NATO troops.


But not everyone was rejoicing.


A UN official reported that Bosnian government troops had fired on a French army helicopter. Four bullets hit the chopper.


Moments after the treaty was signed, explosions rocked two frontline areas in snow-bound Sarajevo -- where the Bosnian Serbs have been protesting in vain against the terms of the peace deal. There were no reports of casualties.


Bosnian government officials also reported that Bosnian Croat militia and Islamic mujahideen forces clashed in central Bosnia. The Bosnian government has promised to expel fighters from countries like Iran within a month.


In a statement issued through the French Foreign Ministry, the leaders of rump Yugoslavia, Bosnia, Croatia, as well as the Bosnian Serbs and the Bosnian Moslem-Croat federation, urged their forces on the ground to "have faith in the future."


French President Jacques Chirac, who hosted the signing in his Elysee palace, said it was time to "turn the page of war."


"This rampage of horror will leave a deep wound in the heart of Europe," he said.


The three presidents from the former Yugoslavia stiffly exchanged handshakes with one another and with major power leaders as they stood up, unsmiling, from signing the treaty.


Clinton, Chirac, German Chancellor Helmut Kohl, Russian Prime Minister Viktor Chernomyrdin and British Prime Minister John Major then signed as co-sponsors.


Bosnia's Moslem-led government long complained it felt abandoned by world powers. Izetbegovic did not applaud the speeches of his fellow leaders, but joined Milosevic and Tudjman in pledging to make the agreement work.


French hopes that the major former Yugoslav republics could agree on an extra deal for the ceremony that would lead to mutual recognition and increase stability did not materialize.