BELGRADE, Yugoslavia -- NATO bombs pounded Yugoslavia for a sixth day as thousands of ethnic Albanians fearing Serb paramilitary forces streamed out of Kosovo Monday in what may be Europe's worst humanitarian disaster since World War II.

One-quarter of Kosovo's populace has now been made homeless since Yugoslav leader Slobodan Milosevic launched the Kosovo crackdown 13 months ago.An ethnic Albanian leader, Fehmi Agani, was executed Sunday, NATO said. Agani, a close aide to ethnic Albanian leader Ibrahim Rugova and one of the negotiators at the failed Rambouillet peace talks, had just attended the funeral of a human-rights lawyer.

Four other prominent ethnic Albanians were also reported executed, NATO said in what it called a "scorched-earth policy" -- including Baton Haxhiu, editor in chief of Koha Ditore, the Albanian-language newspaper in Kosovo's capital of Pristina.

The newspaper's publisher, Veton Surroi, and Rugova both have gone into hiding in fear of their lives, NATO officials reported.

NATO said refugees were arriving at the Albanian border at the rate of 4,000 an hour Monday, straining the already desperate resources of one of Europe's poorest countries.

"We are trying to stop this catastrophe and stop this killing," NATO Secretary-General Javier Solana said after a meeting with the European Union's outgoing commissioner for humanitarian affairs.

The Albanian prime minister appealed Monday to his countrymen to take in the refugees, most of whom were carrying their only possessions by hand and some without even identity documents -- taken away, the refugees said, by Serb authorities at the border.

"It's almost as if their identities are being canceled out," NATO spokesman Jamie Shea said Monday at a news briefing in Brussels, Belgium.

Some 80,000 to 100,000 Kosovo Albanian refugees have arrived in northern Albania, more than double the rate of a few days earlier, the U.N. relief agency said Monday. Thousands more headed west to Montenegro and southeast to Macedonia.

"Are you American?" Nejmije Kelmendi, 50, asked an Associated Press photographer as she trudged up a steep mountain road near Pec in southwestern Kosovo, accompanied by her two daughters. "Tell NATO that Pec is burning, and where are the ground troops?"

NATO seemed to back up the accounts of destruction, saying Monday that Pec was "substantially destroyed."

Yugoslav officials remained defiant, saying NATO's "shameful" attacks were only inflaming the crisis in Kosovo, where ethnic Albanian rebels have been fighting for independence the past 13 months from Serbia, the main Yugoslav republic.

More than 2,000 people have died and a half-million others made homeless since the clashes began in Kosovo last year.

NATO's assault is aimed at getting Milosevic to accept a peace plan that calls for 28,000 troops in Kosovo, including 4,000 Americans.

Asked Monday whether the NATO mission was succeeding, Shea said: "Yes, we are being effective. Yes, the mission is working."

White House spokesman Joe Lockhart said President Clinton remains opposed to using ground troops to supplement the airstrikes, despite growing calls for him to do so.

As Clinton worked to maintain allied support for the broadening air offensive, the White House insisted Monday that NATO airstrikes were not responsible for inflaming ethnic hatred there.

Lockhart said U.S. officials believed Milosevic would aggressively repress Kosovar Albanians, "based on his past actions and what he was doing," regardless of whether NATO carried out the bombings.

"If you look at what's going on at the borders, you have what appears to be a textbook definition of ethnic cleansing," Lockhart said. "We knew that he was going to do this. . . . And we faced a choice between doing something and doing nothing."

Meanwhile, Russia's prime minister, Yevgeny Primakov, announced plans to go to Belgrade on Tuesday in a new bid to end the crisis. Russia, which has cultural and historic ties to Serbia, has strongly opposed NATO's air campaign against Yugoslavia.

NATO spokesman Air Commodore David Wilby said the latest air attacks were against Serb and Yugoslav units involved in atrocities.

"I can tell you that from yesterday we were getting extremely heartening news from sources, which said there was evidence that our campaign was beginning to work and was beginning to disrupt them (the Serbs) and was beginning to worry them," Wilby said in an interview with NBC's "Today" program.

Speaking from NATO headquarters in Brussels, Wilby said the second phase of the military alliance's campaign would focus on the Serb military but would still continue to look at the Serbs' integrated air defense system.

Asked if there were any plans to move NATO ground troops into Yugoslavia, Wilby said there were none.

As daylight strikes got under way Monday, an A-10 "Warthog" ground-attack plane was seen taking off from Aviano Air Base, Italy. The A-10 is a low- and slow-flying tank-killer aircraft that could be used to strike Serb ground forces in Kosovo.

Serbian state-run television repeatedly showed video of a raging fire in the center of Kosovo's capital of Pristina that it said was set off by a NATO missile attack on a police building. Air raid sirens went off in Belgrade, the Yugoslav and Serbian capital.

Rather than restraining the Serbs, however, the attacks appeared only to have intensified their anger at the ethnic Albanians, who made up 90 percent of Kosovo's 2 million inhabitants before the Serbian crackdown.

"The pattern that emerges (from their accounts) is paramilitary forces arriving, rounding people up and telling them at gunpoint to go," said spokesman Kris Janowsky of the U.N. High Commissioner for Refugees in Geneva. "So we are seeing officially sanctioned ethnic cleansing of the Albanian population in Kosovo."

Refugees said Serbs wearing black masks forced them out at gunpoint.

Yugoslav authorities closed at least one crossing point into Albania for several hours Monday, erecting concrete barriers along the main road from the Kosovo city of Prizren to the Albania town of Kukes. It was unclear if other crossing points were also sealed.

Along Kosovo's border with Yugoslavia's smaller republic of Montenegro, thousands of Kosovo Albanians were trying to cross Monday. Police were charging $60 per car to allow refugees to cross.

A 24-year-old refugee from the Suva Reka area of southern Kosovo told a reporter in Albania that when NATO airstrikes began, Serb police "came to our village and told us to go to America, go to NATO and they will help you."

Shea said the situation was on the brink of a major humanitarian disaster, unprecedented since World War II. More than a half-million Kosovars have been uprooted by the crisis, NATO said -- the biggest population shift in Europe since 1945.

But Bratislava Morina, the Serb refugee commissioner, called such accusations propaganda.

"There is no humanitarian catastrophe in Kosovo whatsoever," she said on state-run Serbian television.

Thousands of Kosovo Albanian refugees massed in Kukes Monday, sleeping in doorways and on sidewalks, wandering aimlessly and begging for help.

International agencies scrambled to bring aid to the refugees, but the situation is chaotic. Traumatized refugees swarmed trucks carrying food.

Belgrade has not announced casualty estimates, although Yugoslav U.N. envoy Vladislav Jovanovic claimed Friday that hundreds of civilians had been killed.

Russia's defense minister reported 1,000 civilians dead, but it was impossible under current conditions to independently confirm any of the casualty figures.

Ramifications from the airstrikes began to widen. Germany and Italy, which both have Kosovo Albanian immigrant populations, braced for an influx of refugees.

Demonstrations against NATO action in Yugoslavia tapered off Monday, after U.S. missions as far afield as Russia, Australia and Canada were rocked by protests over the weekend. Less-violent rallies were reported in Romania, Greece and Israel.

Meanwhile, the pilot of the first plane NATO lost in the assault on Yugoslavia returned to Aviano after being rescued by the allies.

The pilot of the F-117A stealth fighter-bomber suffered cuts but no serious injuries when the plane went down, said U.S. Air Force Capt. Edward Thomas.

"He got off the airplane and was greeted by a large crowd of friends and squadron mates, commanders and subordinates. It was better than watching the Super Bowl," Thomas said.

At Camp David, Md., Sunday night, Clinton spoke by phone with the pilot, Lockhart said. The president also spoke with a few of the pilot's rescuers. He returned to the White House Monday for briefings by his foreign policy advisers.