SARAJEVO, Bosnia-Herzegovina (AP) _ Bosnian Serbs promised Tuesday to end their blockade of Sarajevo, lending an upbeat note to a day when new fighting and diplomatic conflict threatened a lasting truce.

The Serbs, however, continued to demand that the government army vacate key positions overlooking Sarajevo. The unspoken threat was clear: Even if the Serbs keep their promise, they can blockade Sarajevo anew unless their demands are met.

If the Serbs open roads linking Sarajevo with other government-held territory, they would be meeting some terms of a planned four-month truce signed Dec. 31.

But a Bosnian Serb statement reiterated demands that government forces, in addition to vacating a demilitarized zone around Mount Igman southwest of Sarajevo, also withdraw from two locations east of the zone. The latter demands are not part of truce conditions.

``Despite the fact that the Muslims (government troops) keep refusing to withdraw their forces from the illegally occupied ground on Mt. Igman and Mt. Bjelasnica, the Serbs are nevertheless going to open up the routes for civilian traffic in and out of Sarajevo by the end of the week,'' the statement said.

That would open two roads _ one linking Sarajevo to government-held areas to the south, the other connecting the city with Visoko, 13 miles to the north, and with areas in central Bosnia.

The roads played a vital role in feeding Sarajevo's 280,000 residents until the Serbs closed them in July.

Serb forces made clear their demands for further government withdrawals stood.

``We shall insist that the Muslims withdraw from the zones on Mt. Igman and Mt. Bjelasnica ... because they would otherwise be putting at risk the entire peace process,'' the statement said.

The problem of rebel Serbs from Croatia fighting in northwestern Bosnia also remained unresolved.

Lt. Gen. Sir Michael Rose, the commander of Bosnia's U.N. peacekeepers, cancelled a trip to Knin, the headquarters of rebel Serb fighters in neighboring Croatia, to seek the Croatian Serbs' withdrawal from the Bihac pocket and unhindered movement of U.N. convoys.

U.N. officials said Croatian Serb commanders refused to meet with Rose. The Serbs, though, said the commanders had previous commitments.

Bosnian radio reported a new offensive Tuesday near Velika Kladusa, a border town in the Bihac area, by a joint force of rebel Serbs from Croatia and Bosnia, aided by renegade Muslims. Maj. Herve Gourmelon, a U.N. spokesman said small-arms fire and 104 artillery and mortar detonations had been registered.

The four-month truce is designed to cool tempers enough to give permanent peace a chance. International negotiators want new talks on a plan that would divide the republic between Serbs and a Croat-Muslim federation.

The plan has problems, however. The Bosnian government accepted it as a take-it-or-leave-it deal, but Serb refusal to sign has forced the ``Contact Group'' of international negotiators to redefine it as a starting point.

That would leave most of its provisions still open to change _ something the Bosnian government opposes.

The plan would give 49 percent of Bosnia to Bosnian Serbs, who have captured 70 percent of the republic since April 1992. The government and its Croat allies would get 51 percent.

At least 200,000 people have died or disappeared in the war.