WASHINGTON — PRESIDENT Clinton's high-stakes gamble in the Balkans appears to be paying off as Serbian leaders agreed to remain engaged in peace talks, even as NATO threatened to launch new airstrikes against rebel Bosnian Serb forces.
''The road is now open for serious, substantive negotiations,'' Assistant Secretary of State Richard Holbrooke declared in Belgrade as he forged ahead with a United States diplomatic initiative to end four years of fighting in the former Yugoslavia.
Mr. Holbrooke's statement came one day after President Slobodan Milosevic of Serbia wrested from Bosnian Serb leaders the power to negotiate on their behalf. This was seen in Washington as a significant breakthrough, since Mr. Milosevic has publicly endorsed the US peace plan. The plan calls for dividing Bosnia among its main ethnic groups while maintaining a unified state.
Despite the apparent diplomatic progress, Western governments sustained military pressure on the Bosnian Serbs as NATO warplanes engaged in ''Operation Deliberate Force'' prowled the skies over Sarajevo seeking fresh Serb targets.
NATO commanders said the operation would continue until the Bosnian Serbs withdrew their heavy weaponry around Sarajevo and allowed free access to the besieged Bosnian capital.
Clinton administration officials readily concede that the strategy of unleashing the largest military operation in NATO's 45-year history at the same time Holbrooke is pursuing peace talks could backfire. But in the end they concluded that the potential gains outweighed the risks.
''There are a number of people who thought that there could be a number of outcomes to this, and one is that we could literally ruin our chances for a diplomatic settlement,'' says a senior Clinton administration official. But administration officials finally concluded ''that if we don't respond now it could have the same effect,'' he said.
The airstrikes began early Wednesday in the aftermath of a Serb mortar attack that killed 38 people outside Sarajevo's main market. The NATO raids marked a dramatic reversal of policy for Western nations, which for more than three years have failed to stick with their commitment to protect Muslim enclaves designed as ''safe areas'' by the United Nations.
The decision to launch the airstrikes has already brought political benefits to President Clinton, whose Bosnia policy has been under sustained attack in Congress.
Senate majority leader Bob Dole (R) of Kansas announced Wednesday that he might postpone a Senate vote to override Clinton's recent veto of a resolution requiring the US to unilaterally exempt Bosnia from a 1991 UN arms embargo.
''I am willing to consider postponing Senate action if the recent Western attacks prove to be part of a new and effective policy which leads to a just and lasting peace settlement,'' Mr. Dole said Wednesday.
An override would represent an embarrassing repudiation of Clinton's foreign- policy leadership as he gears up for a difficult 1996 reelection battle. It would also threaten NATO unity, even as Western leaders have closed ranks on the use of force in Bosnia for the first time.
The US strategy has also been bolstered by a grudging acknowledgment by Russian President Boris Yeltsin - apparently reflecting the positive diplomatic developments in Belgrade - that the airstrikes against Bosnian Serb targets have been justified. Mr. Yeltsin initially condemned the raids as a ''cruel bombardment.''
Up to press time yesterday, NATO planes from the US, Spain, Britain, and the Netherlands had launched more than 300 sorties, using 1000- and 2000-pound bombs and laser-guided missiles to attack Bosnian Serb targets including artillery positions, command-and-control centers, ammunition dumps, and communications facilities.
The raids were supplemented by artillery attacks by a UN Rapid Reaction Force comprising French, British, and Dutch troops deployed near Sarajevo.
Adm. Leighton Smith, commander of NATO's southern flank, told reporters yesterday that NATO air attacks and UN artillery strikes would continue until the Bosnian Serbs opted for diplomacy instead of further military action against Bosnia's Muslims and Croats.
A senior US military officer said NATO air operations were being hampered by cloudy conditions over Sarajevo.
Admiral Smith said the attacks had ''clearly reduced the effectiveness'' of the Serbs' integrated air-defense system over southern Bosnia.
Yesterday, Holbrooke flew from Belgrade to Zagreb, the capital of Croatia, for further talks with Bosnian and Croatian leaders. The peace plan he is carrying is a modified version of a proposal drafted last year by a ''contact group'' of US, French, Russian, British, and German diplomats.
The Bosnian Serbs, who control 70 percent of Bosnia, rejected the plan, which gives them only 49 percent of the country and denies them independence.
In talks with Holbrooke Wednesday in Belgrade, Milosevic announced that he would play the lead Serb role in future talks. ''After 16 months of arguing about who speaks for the Serbs, that issue has been resolved and the decks are now cleared for serious negotiations,'' Holbrooke said in Zagreb on Thursday. But he cautioned against ''false optimism.''