BELGRADE, Yugoslavia (AP) _ Yugoslav authorities sealed off Kosovo's main border crossings Wednesday, preventing ethnic Albanians from leaving as the wave of refugees approached the half-million mark. As NATO stepped up its airstrikes, a Cypriot mediator sought freedom for three captured U.S. soldiers.

Two loud explosions could be heard near midnight Wednesday in the center of Belgrade, and smoke could be seen rising in the direction of the army headquarters.

Earlier Wednesday, NATO planes and cruise missiles struck military targets in Kosovo and inflicted heavy damage elsewhere in Yugoslavia.

Albania's parliament approved NATO plans to send in 24 U.S. Apache attack helicopters _ a move that should bolster the alliance's firepower against the Yugoslav tanks and armor that have driven ethnic Albanians from their homes in Kosovo.

After forcing more than 400,000 refugees out to neighboring countries, Yugoslav authorities closed the main exit route on Wednesday without explanation, forcing tens of thousands of people back toward the burned villages they had been escaping from.

German Defense Minister Rudolf Scharping warned that the Yugoslavs may be planning to use the civilians in Kosovo as ``human shields'' against NATO attack.

In Morini, the crossing point for Kosovo refugees into Albania, the flow of cars and tractors suddenly stopped at 3 a.m., witnesses said. Yugoslav border guards could be seen laying what appeared to be mines and digging fortifications just inside their territory.

``The refugees ... were told to return to their places of residence _ whatever is left of those places,'' said Doran Vienneau of the Organization for Security and Cooperation in Europe, which has been monitoring the border.

Scharping said Yugoslav forces had begun forcing ethnic Albanians back from the border areas into the province. He showed aerial photos that he said showed Serb tanks surrounding a Kosovo village, separating the men and women, and then opening fire on houses.

The former president of Cyprus, Spyros Kyprianou, began a mission to win the release of three American soldiers seized along the Yugoslav border with Macedonia on March 31. Cyprus, which is not a NATO member, has historically had close ties with Yugoslavia.

Kyprianou flew to Athens, where he said he was ``waiting for the green light from Belgrade'' before going to Yugoslavia on Thursday.

He said he believed the release of the soldiers was imminent.

Sandy Berger, President Clinton's national security adviser, was cautious about the mission but said ``we certainly would welcome any release.''

Another nightmare was brewing for the tens of thousands of Kosovo refugees who had made it across into neighboring Macedonia only to be trapped for days in squalid conditions.

With frightening efficiency, Macedonian officials emptied the border city of Blace before dawn Wednesday, hustling thousands of refugees onto buses so quickly that some were separated from their families.

Macedonia, which has been criticized for conditions at the makeshift camp, said it shipped 10,000 refugees to Albania and moved 25,000 others to NATO tent camps further in-country.

Aid workers suggested the Macedonians used force to move out the refugees, and said 10,000 refugees from the camp were unaccounted for.

``Most people here don't know what their fate is going to be. They don't even know where their relatives are, where their mothers are,'' said Nebi Qena, 21, after being moved from Blace to a NATO-run camp.

Macedonia has been flooded with more than 130,000 refugees since NATO airstrikes began March 24, and it has castigated the United States and its NATO allies for failing to stem the tide.

At NATO headquarters, Air Commodore David Wilby said alliance aircraft struck before dawn Wednesday at dozens of military targets and Yugoslav forces in and around Kosovo.

He announced NATO's ``first major breakthrough against armored forces in the field,'' saying ``we were able to locate and attack several units.'' In one attack, he said, allied planes dropped weapons on a column of seven to 12 vehicles.

In Washington, Air Force Maj. Gen. Charles Wald, vice director for strategic plans and policy for the Joint Chiefs of Staff, said the low-flying ``tank killer'' A-10 aircraft hit Serb forces in Kosovo overnight.

The Yugoslav news agency Tanjug said at least 10 people were killed and eight seriously injured in an airstrike Wednesday on administrative buildings in Kosovo's capital of Pristina.

Television footage of Pristina showed huge mounds of rubble, twisted steel and broken glass.

NATO is trying to force Milosevic to withdraw Serb special police and military units from Kosovo and accept a three-year interim autonomy agreement that would be policed by 28,000 NATO troops, including American soldiers.

Ethnic Albanians made up 90 percent of the 2 million prewar populace of Kosovo, a province in Serbia, the main republic in Yugoslavia. Milosevic's crackdown on ethnic Albanian separatists began 14 months ago.

NATO spokesman Jamie Shea told reporters in Brussels, Belgium, that NATO was ``deeply preoccupied'' by reports of atrocities being committed against civilians and that in Kosovo 50 villages reportedly have been burned since Saturday.

He also said three mass gravesites been identified, based on refugee testimony.

On Wednesday, the rebel Kosovo Liberation Army claimed Serb forces massacred at least 51 civilians, including 12 children, during a raid on the village of Celina, southwest of Pristina, hours after the airstrikes began.

In Washington, State Department spokesman James P. Rubin said nine Serb commanders have been linked to atrocities in Kosovo and could face war crime prosecutions.