U.S. Envoys Exploit Balkan Shifts : Diplomacy: Americans shuttle among capitals to push initiative. Croatians press assault on Serbs.

U.S. diplo mats, hoping that shifting battlefield fortunes will improve chances for peace, shuttled between Serbia and Croatia on Friday to push an initiative that Croatia’s president said could produce a deal within weeks.

But the main warring sides in Bosnia-Herzegovina--the Muslim-led government and rebel Serbs--adopted a more stubborn attitude a day before the U.S. delegation was expected in Sarajevo, the Bosnian capital.

Croatian troops, meanwhile, pressed an assault on rebel Serbs. Croatia claimed to have captured Drvar, a strategic town across the border in Bosnia, although the Serbs denied it, and 10,000 Croatian troops massed near Dubrovnik, a resort on Croatia’s southern coast.

“The situation [in Dubrovnik] is extremely tense,” said Chris Gunness, a U.N. spokesman in Zagreb, the Croatian capital. “We believe the offensive actions could be launched within days.”

Croatian commanders have vowed to keep Serb gunners in the town of Trebinje, just across the border in Bosnia, from firing at the walled medieval city of Dubrovnik. As shells fall closer to the center of town, carpenters have boarded up historic statues.

U.S. Assistant Secretary of State Richard Holbrooke refused to comment on the peace proposal. He held talks Friday with both Serbian President Slobodan Milosevic, the regional powerbroker, and Croatian President Franjo Tudjman. Holbrooke was to continue to Sarajevo to meet with government leaders today.

Media reports have said the plan involves a land swap in which the Bosnian government would give up the eastern Gorazde enclave and land around a Serb-held corridor in the north in exchange for territory around Sarajevo.

But Bosnian Prime Minister Haris Silajdzic said Friday that Holbrooke had assured him the plan did not include such a land swap.

The plan reportedly also would lift economic sanctions against the Serb-led rump Yugoslavia and provide international aid to rebuild Bosnia. If either side refuses to participate, the plan would allow arms sales to its enemy.

Tudjman told a news conference in Zagreb that the U.S. proposal was aimed at creating “a new, stable international order in southeast Europe.”

“We would like these initiatives to work out, and in a few weeks’ time to have a peaceful solution,” Tudjman said.

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In Bosnia, the fighting continued. A rifle grenade hit a packed bus in a Sarajevo suburb, killing a 13-year-old girl, the Bosnian Health Ministry said. The girl’s parents and at least 12 other civilians were wounded.

Late Friday, Bosnian President Alija Izetbegovic declared that the government must not get less than the 51% of the country’s territory envisaged in a previous peace plan, which would give 49% to the Bosnian Serbs. The government now holds 30%.

Bosnian Serb television, meanwhile, showed footage Friday night from the western town of Drvar, which Croats said their troops had captured.

The Bosnian Serb army chief of staff, Manojlo Milovanovic, said the town was still in Serb hands Friday afternoon.

A Croatian victory in Drvar would put Zagreb’s army within striking distance of Bosanski Petrovac, the last Serb-held town in western Bosnia separating the Croats from their Bosnian army allies in the north.

Gunness, the U.N. spokesman in Zagreb, said there was new evidence of apparent atrocities by Croatian troops as they drove the Serbs out of southern Croatia two weeks ago.

He said U.N. workers found four bodies Wednesday in Zagrovic, 120 miles south of Zagreb and near the former rebel stronghold of Knin. Three of the bodies had bullet holes in the head.

U.N. workers also saw what appeared to be mass graves in Knin, Gunness said. He said that burning and looting of abandoned Serb houses was continuing.

Tudjman has denied that his troops have committed atrocities.