BELGRADE, Yugoslavia -- The Bosnian Serbs earned a second 72-hour reprieve from NATO bombing yesterday by pulling back heavy weapons around Sarajevo, but a continuing Bosnian army and Croat offensive in northwest Bosnia raised fears that recent tentative moves toward peace could unravel.
United Nations and NATO commanders, who had stopped the bombing of Bosnian Serb positions on the condition that the Serbs withdraw most of their big guns on the heights around Sarajevo, the Bosnian capital, said that the Serbs had made "substantial progress" and that the bombing pause would be extended for another 72 hours.
But the easing of tensions around Sarajevo coincided with heightened concern about the most effective Bosnian government offensive since the start of the war. The advancing troops moved yesterday to within 25 miles of Banja Luka, the Bosnian Serb stronghold in northwest Bosnia.
U.N. and U.S. officials said they feared that the assault could draw Serbia directly into the conflict.
Bosnian government troops, backed by their Croatian allies, have seized about 1,600 square miles from the Bosnian Serbs in the past week. Tens of thousands of Serbian refugees, fleeing the advancing Bosnian government troops, have streamed into Banja Luka, which was already flooded with Croatian Serb refugees expelled from Croatia.
Bosnian Serb leader Radovan Karadzic, in an unusually gloomy statement, told the Russian news agency Itar-Tass yesterday that his forces were taking "heavy losses." Other Bosnian Serb officials said they were hastily building barricades and trenches to defend Banja Luka.
In Washington, a Clinton administration official said that the United States had strongly discouraged the Bosnian government from attacking the city of about 600,000. A threat to Banja Luka, the official said, could well prompt Serbian President Slobodan Milosevic to send Yugoslav army troops into Bosnia.
U.S. negotiators scrambled yesterday to try to fit the swiftly changing front lines between the hostile parties into a cease-fire agreement or peace plan.
"The next few days and hours may determine the fate of the war," U.S. Assistant Secretary of State Richard C. Holbrooke said as he arrived in Sarajevo for talks with Bosnian government officials.
The U.S. peace initiative, accepted by the warring factions, is intended to give a confederation of Bosnian Muslims and Croats 51 percent of Bosnia and the Serbs 49 percent.
The rebels held close to 70 percent of Bosnia a few weeks ago. But the Bosnian government and Croat offensive has reduced this to about 55 percent.
While the reduction in territory under Bosnian Serb control would make it easier to divide the land along the lines of the 51-49 formula, it could also embolden the Bosnian government to regain on the battlefield what it has so far been denied on the negotiating table.
Mr. Holbrooke met for seven hours with Mr. Milosevic in Belgrade and talked with Croatian President Franjo Tudjman in Zagreb before flying to Sarajevo to meet Bosnian President Alija Izetbegovic.
He is scheduled to hold more talks in Belgrade today. The U.S. team will meet tomorrow with Mr. Izetbegovic and Mr. Tudjman in Zagreb, and plans to return to Washington afterward.
The Bosnian Serbs, who agreed Thursday to withdraw most of their heavy weapons, have towed 60 pieces of heavy artillery, including nine 105 mm howitzers and five 122 mm artillery pieces, out of the 12.5-mile exclusion zone around Sarajevo, U.N. officials said. They have also removed tanks, anti-aircraft guns and rocket launchers.
NATO mounted a two-week bombing campaign of Serb positions to end the siege of Sarajevo before suspending strikes Thursday to give the Serbs a chance to pull back their guns.
After U.N. commanders complained about the terms of the agreement, Mr. Holbrooke said yesterday after talks with Mr. Milosevic that the Bosnian Serbs also would withdraw mortars with bores of 82 mm and cannons with bores of 100 mm, not just weapons of larger bores.
Gen. Dragomir Milosevic, the Bosnian Serb commander of the troops encircling Sarajevo, said his force had withdrawn about half of the some 300 artillery pieces and tanks that the United Nations estimates are positioned around the besieged capital.
But much now depends on curbing the fighting around Banja Luka, a city short of supplies and swamped with families displaced by the advance.
"The situation in the city itself is one of complete chaos with cars, tractors and horse carts in every street," said U.N. refugee spokesman Mans Nyberg. "All access roads to the city are blocked."
Meanwhile, air traffic continued into the Sarajevo airport, reopened to aid flights. And relief aid is now passing down two roads into the capital.