But their advance into the city has been complicated by the presence of Russian troops, who unexpectedly entered Pristina on Friday.

The Russians took up positions at the airfield, which had been designated as the headquarters for the United Kingdom's contingent in the K-For peacekeeping force.

The BBC's Mike Williams follows the convoy towards Pristina

The 700 UK troops reached the edge of the city at about 1500 local time (1300 GMT), after crossing the border at daybreak.

UK commanders are currently negotiating with their Russian counterparts over control of the airfield.

The Russians have painted the name of K-For on their military vehicles - which a BBC correspondent in Pristina says would imply they have accepted the command of K-For's leaders.

The Russians' role in the peacekeeping force is also under discussion at a diplomatic level.

Talks in Moscow between US envoy Strobe Talbot and Russian officials were temporarily adjourned on Saturday, apparently without reaching a conclusion.

Mr Talbott is believed to have refused any modification to the UN-backed formula for peacekeeping in Kosovo.

First phase complete

For three hours after dawn, wave after wave of helicopters ferried about 1,500 British troops across the border from the former Yugoslav Republic of Macedonia in to Kosovo - completing the first phase of Nato's operation to secure control of the Serbian province.

Their task was to secure the hillsides and the main road leading north to Pristina.

On the way, they were searching for mines and booby traps left by the retreating Serb forces.

The size of the convoy slowed them down, but no major incidents were reported along the way.

In one minor delay, Yugoslav army officers manning a checkpoint on the main road said they needed more time to retreat from their positions.

Nato forces did not compromise, and the Yugoslavs let them proceed after about 30 minutes.

Earlier, UK soldiers came across more than 30 Serb military and paramilitary troops, and disarmed them.

Albanians celebrate, Serbs flee

The deployment is one of the biggest military operations in Europe since World War II.

Ben Brown reports: "Nato can afford to take nothing for granted"

It is intended to help thousands of Kosovo Albanian refugees return home, though they have been warned they will have to wait until the province is made safe.

There were scenes of celebration in the refugee camps in the Former Yugoslav Republic of Macedonia as the Nato forces moved into Kosovo.

Several thousand Serbian refugees are reported to have arrived in Montenegro, after fleeing Kosovo.

Many Serbs in the province feel that Nato cannot guarantee their safety after the withdrawal of Yugoslav forces.

UK, France lead the way

British and French troops spearheaded the initial thrust, with German, American and Italian forces due to move in later. The multi-national force for Kosovo will eventually number around 50,000.

British paratroops and Gurkhas on board Chinook helicopters were the first Nato soldiers to enter the province.

Beneath them, thousands of troops began crossing the ground border at Blace.

Although the international peacekeeping operation was launched with the authority of the United Nations, most of troops are from the Nato nations which carried out the air campaign against Yugoslavia.

French troops, led by mine clearing experts from the French Foreign Legion, moved into Kosovo along two lines of advance several kilometres apart and to the east of Blace.

(Click here to see an animated map showing timetable of Serb withdrawal and Nato's planned movements)

The BBC's Paul Wood in Kosovo: "Jubilation among KLA fighters"

Paratroopers and Gurkhas are securing the landing zones and high ground above the road to Pristina.

In the hours before the troop movement, a British transport plane crashed on a military airfield near the Albanian town of Kukes, injuring one person. Advance upstaged

But the Nato advance was upstaged by the Russian advance into Pristina. The move came as Nato was still negotiating with Moscow about its role in the peacekeeping effort.

The Russian Foreign Minister Igor Ivanov immediately ordered the Russian troops back out again. He said they did not have government authorisation to enter Kosovo.

But the BBC's Rob Parsons in Moscow says the Russian troops appear to have been acting on orders from the Defence Ministry.

He says Russian generals are not happy with the agreement over Kosovo, and the events of the past 24 hours reflect that.

The BBC's Rageh Omaar: "They are not moving across the border yet"

A Russian solder said they had come to "prepare the airport for the arrival of Russian aircraft and then they would depart". He expected the unit would be there for two weeks.

• Russian troops camp in Pristina
• The push into Kosovo in pictures
• UK leads Nato into Kosovo
• Kosovo Serbs pack up and leave
• K-For: The task ahead