To the cheers of ethnic Albanians and the sullen stares of Serbs, advancing NATO peacekeeping troops reached the Kosovo capital of Pristina today and immediately confronted Russian soldiers over control of the city's airport.

Led by the British and French, the first NATO forces moved in waves across the Macedonian border into the Serbian province throughout the day, quickly establishing authority over the main highway into Pristina. They encountered some resistance from Yugoslav army troops that had not yet withdrawn from Kosovo, but the encounters were resolved peacefully.

The atmosphere nonetheless remained tense, and as the first day of NATO's occupation of Kosovo ended, the war-torn province was a volatile stew of competing NATO, Russian, Serbian and ethnic Albanian guerrilla forces, and NATO had yet to establish complete command.

The first British forces to make the 40-mile journey from Macedonia to Pristina moved quickly to the airport, only to find that it already was occupied by several hundred Russian troops. The Russians, bent on establishing a role in the peacekeeping operation, caught the West by surprise early today when they sent an armored column from Bosnia to Kosovo, beating NATO in despite assuring the Western allies that Russia troops would stay out.

The action led to frantic consultations over Russia's place in what U.S. officials have depicted as a NATO-run operation. The troops' move into Kosovo was endorsed today by Russian President Boris Yeltsin -- just hours after Foreign Minister Igor Ivanov had said it was unfortunate and that the Russian soldiers had been ordered to leave Kosovo immediately.

The White House originally welcomed Ivanov's statement that the Russians would withdraw, but today officials in Washington and at NATO headquarters in Brussels sought to play down the significance of the Russian move, saying it would not threaten the peacekeeping mission.

At Pristina's airport, however, a British pool reporter described a tense encounter when Russian forces, supported by Serbs, attempted to bar British troops from the site. At one point, some 200 British army vehicles, including a tank and 400 soldiers, massed on the road to the airport, according to a British pool report distributed by the Reuters news service.

The Russian and Serbian forces refused to back down, although the Russians eventually permitted a small contingent of British troops into the airport, which was one of the prime first targets for NATO in taking control of Kosovo. After three hours of negotiations between British and Russian officers, a tentative deal granted the British control of the southern sector of the airport, leaving the Russians with the rest.

When reporters later tried to attend a news conference at the airport by Lt. Gen. Mike Jackson, the British commander of the peacekeeping force, they were turned away by Serbian police, who blocked access to the site. Members of the small British press pool reported that Russian armored personnel carriers roared up and down the runway, effectively drowning out Jackson's remarks.

In Washington, Gen. Henry H. Shelton, chairman of the Joint Chiefs of Staff, said that the airport is under "a coordinated occupation" between the Russians and NATO, "and the final details are going to be worked out."

While it took the small number of Russian troops stationed in Bosnia less than a day to move through Yugoslavia proper and into Kosovo, the much larger NATO deployment from Macedonia was slow and ponderous. Today's two-pronged advance by British and French troops, along with a small contingent of American advance teams, was held up by mine-clearing operations and a two-hour standoff with retreating Yugoslav army units. The first U.S. troops crossed into the province about 10 hours after the operation began, when nine armored Humvees from the 82nd Airborne Division rumbled over the border from Macedonia in a convoy that included German soldiers.

As the day wore on, it was clear that the balance of power in Kosovo -- a province of Serbia, Yugoslavia's dominant republic -- was rapidly shifting away from the Yugoslav army and Serbian police and paramilitary units who have terrorized the province's predominantly ethnic Albanian population. With some exceptions, both Serbs and ethnic Albanians appeared ready to cooperate with the NATO troops in implementing the peace agreements hammered out over the past week. British troops disarmed 30 Serb paramilitary militiamen without incident, the Associated Press reported.

As the NATO soldiers streamed into Kosovo as the advance contingent of a 50,000-strong peacekeeping force, an estimated 40,000 Yugoslav troops continued their withdrawal to the north. Departing with them were thousands of Serbian civilian refugees, their belongings piled high on tops of cars and farm wagons in scenes at least superficially reminiscent of the mass ethnic Albanian exodus in March and April after Belgrade responded to the NATO bombing campaign with a brutal wave of expulsions.

"This is what happened to the [ethnic] Albanians. It's our turn now," said Slavolub Jovanovic, 20, a student from the region of Suva Reka in southern Kosovo, driving north in a convoy of tractors that included three generations of his family. "How can we believe NATO when they bombed us?"

British officers leading the main NATO advance into Kosovo, along the Skopje-Pristina road, said they had encountered little resistance. But an incident near the village of Kacanik, 10 miles north of the border with Macedonia, illustrated the kinds of problems that NATO is likely to face as it wrests control of Kosovo from the Serbs.

A small, north-bound advance unit of Gurkhas, small mountain men from Nepal who are a mainstay of the British army, had run into a column of Yugoslav army trucks that wanted to move south to retrieve equipment hidden in the woods. The Gurkhas, under orders not to let armed men down the road, blocked the way with three combat reconnaissance vehicles. A Yugoslav army major threatened to shoot his way through the blockade.

"Please be patient," pleaded the Gurkhas' British officer, Capt. Fraser Rea, as he radioed for reinforcements. "The last thing I want is an escalation."

Eventually, a senior Yugoslav officer, Gen. Jan Djakovic, arrived in a white Mercedes convertible with no license plates and demanded to speak to the senior British commander. "You are breaching the military-technical agreement," he sputtered, referring to a seven-page document signed last Wednesday between NATO and the Yugoslav army in Macedonia.

A British brigadier, Adrian Freer, arrived to negotiate with Djakovic through an interpreter. First they glared at each other. But soon they were patting each other on the back. The Serbs were permitted to retrieve their equipment, and the British armored column rumbled northward toward Pristina.

A bluff general eager to smooth over disputes with the Serbs, the Russians and the ethnic Albanian Kosovo Liberation Army, Freer said that he was on his way to Pristina airport to "make love" to Russian commanders. "Obviously there have been some spats happening at very high levels, but we have to work with them," he said. He took an equally conciliatory attitude to rebel army units advancing into Kosovo behind the peacekeeping force, saying that it was premature to disarm them.

French troops advancing along another route into Kosovo were delayed along the Macedonian border by a minefield that French officers said the Serbs had failed to report. A French mine-clearing team was dispatched to the area to defuse the mines, while some French infantry units detoured around the obstacle to fan out into the province.

As the country that will be contributing the largest contingent in the peacekeeping force, Britain was given the honor of leading NATO troops into Kosovo, although the prize was diminished somewhat by the early arrival of the Russians. U.S. troops were "bringing up the rear," said Lt. James Blount of the Royal Household Cavalry, a smile lighting up his face, betraying the often-unstated rivalry between the longtime NATO allies.

The 7,000-strong U.S. contingent, including about 2,000 Marines, is expected to be based near the town of Gniljane in eastern Kosovo. A town of 70,000, 70 percent of whom are ethnic Albanian, Gniljane escaped the worst excesses of mass expulsions during the war. Serbs say they feel more secure there than in western regions of Kosovo, and do not appear to be leaving in large numbers, at least not yet.

Even so, Gniljane is a depressing place, with many looted businesses and burned-out homes. The bus depot was hit by NATO in a raid on a nearby military facilities. Local Serbs blame the attacks on ethnic Albanians on paramilitary groups from Serbia proper who arrived in Gniljane during the first days of the war. Ethnic Albanians say there is some truth to that claim, but it is not entirely convincing.

"You are who your friends are," said a 20-year-old ethnic Albanian man. "If you are with people who are bad, then you are bad yourself."

The Serbian manager of the Hotel Europa, Rela Nesic, said he was eager to welcome American guests. He blamed Belgrade as much as Washington for the destruction caused by the war. "This was a stupid war. It was stupid for anyone to think that we could win a war against such a force as NATO," he said. His hope is that the Americans will defend the Serbs from revenge by the secessionist ethnic Albanian guerrillas. "If that happens, then everything will be fine."

PEACEKEEPERS MOVE IN

NATO peacekeepers began flooding into Kosovo yesterday, and an advance British convoy reached Pristina, where it met with Russian troops already encamped there. The eventual Russian role in the 50,000-strong peacekeeping force was still being discussed.

PRISTINA: Several hundred Russian soldiers took up positions in the northern part of Pristina airport.

The first British troops to reach Pristina encamped at the south end of the airfield.

FRENCH TROOPS: Advance troops of the 7,000-strong French contingent entered here. The convoy was delayed by discovery of a minefield just inside Kosovo.

KACANIK PASS: Forward British troops entered Kosovo here on their way to Pristina, where the peacekeeping force's headquarters will be. Heavy equipment was ferried in by helicopter.

KACANIK PASS: A small advance party of U.S. Marines entered Kosovo with British troops. Later, units of the U.S. Army's 82nd Airborne Division crossed into the province.

MORINA PASS: Most of the 8,500-strong German contingent was expected to move into its western sector today.

SOURCE: U.S. Department of Defense

DEVELOPMENTS IN THE BALKANS

* British and French forces led thousands of NATO troops across the Macedonian border into Kosovo on the first day of a military operation to restore peace there.

* The first British units to arrive in Pristina, the provincial capital, confronted Russian soldiers already at the city's airport. After a standoff, they reached a tentative agreement to share control of the site.

* Russian President Boris Yeltsin endorsed the deployment of several hundred Russian troops in Pristina, only hours after Foreign Minister Igor Ivanov said it was a mistake and said they had been ordered to leave.

* The first U.S. troops crossed into Kosovo about 10 hours after the operation began. Nine armored Humvees from the 82nd Airborne Division rumbled over the border from Macedonia in a convoy.

* As the NATO forces moved into Kosovo, Yugoslav troops and Serbian police forces continued their withdrawal to the north, along with thousands of Serbian civilians.

CAPTION: Reqica Shaban, right, greets NATO troops with a triumphant gesture hours after departing Yugoslav troops who occupied his house in Urosevac set it ablaze.

CAPTION: Under hovering helicopters, British troops cross the bridge at Kacanik, Yugoslavia, into Kosovo in biggest military operation in Europe since World War II.

CAPTION: As British vehicles roll into Kosovo, a uniformed guerrilla of the rebel Kosovo Liberation Army watches from the roadside as the column advances.