Belgrade has ordered its military and special police to mount large-scale attacks in Kosovo to recapture pockets held by ethnic Albanian rebels and reopen roads the separatist rebels have blocked for weeks, western diplomats and military officials said on Friday.

The escalation of the fight against the rebels came as Richard Holbrooke, the U.S. special envoy, left Pristina empty-handed after four days of shuttle talks with the Yugoslav president, Slobodan Milosevic, and ethnic Albanian leaders. "We all agree that we are at a dangerous moment and our goal remains to prevent the fighting that's already going on from escalating to a general war," Holbrooke said.

There appeared to be little left that western diplomats could do to prevent the assault, given the intransigence of the insurgents and determination of Milosevic to retake the main arteries in the province.

Diplomats said they feared the attacks, which could begin within hours or days, might lead to further fighting beyond the borders of the province and could usher in a low-intensity war that could last for years.

Holbrooke had hoped to secure an agreement from the Kosovo Liberation Army rebels to take down barricades along main routes in the province. He had planned to use the reopening of the roads as leverage to get Milosevic to call off an attack against the insurgents. But the rebels, who say they will fight until they have achieved their goal of an independent state, rebuffed Holbrooke.

The rebels' lack of a central command made it difficult for Holbrooke to be sure his message had reached the top rebel authorities. He asked Adem Demaci, a militant political leader who openly supports the rebel movement, to pass on a message for him, but Demaci was unsure whom to contact, Western diplomats said.

On his visit here, Holbrooke, reiterating demands made by the United States and its allies, called for a resumption of talks between Belgrade and the pacifist ethnic Albanian leader, Ibrahim Rugova, that were broken off by the ethnic Albanians because of the fighting. He also called for all checkpoints to be removed, for Serbian special units to be withdrawn and for the approximately 100,000 ethnic Albanians driven from their homes in the attacks to be allowed to return to their villages.

Washington has called on Belgrade to grant the ethnic Albanians the autonomous status it revoked eight years ago but does not support the drive by the insurgents to create an independent state.

There were numerous indications on Friday that an attack was imminent.

Yugoslav MiG-21 fighter planes roared in pairs down the runway of the Pristina airport. The military has set up 30mm anti-aircraft guns and SA-9 Gaskin surface-to-air missiles around the airport and on hilltops. There has been a concentration of tanks, armored personnel carriers, and heavy artillery on the edge of the rebel-held pockets and large-scale movement of troops along the roads outside of the provincial capital.

"The signs are there of a pending attack," said a senior western military official. "It will take the Serbs a few hours to roll into these areas and smash through the roadblocks. The rebels don't stand a chance. But this attack will trigger a wider, deadlier war, bringing the conflict to the cities, creating small, mobile bands of armed men that could see fighting here last for years. The Serbs will make some short term gains, but this assault could mean they have lost Kosovo."