BELGRADE, Yugoslavia (AP) _ NATO took its air war to the heart of Belgrade and the hills of Kosovo, while a former president of Cyprus said today he was hopeful he could win the release of three U.S. servicemen held in Yugoslavia.

In the Yugoslav capital, loud explosions rocked a densely populated neighborhood in the city center shortly before midnight Wednesday, and smoke rose from the direction of the army headquarters. An army building near the headquarters was hit, a local resident said by telephone.

Despite the strike on Belgrade, overnight attacks on population centers were far less intense than on previous nights since the air campaign began, according to Yugoslav media reports. Pentagon officials said the latest wave of airstrikes instead focused on police and army units scattered in rural areas of Kosovo, a province of Serbia, Yugoslavia's dominant republic.

Small Yugoslav units have been blamed for most of the atrocities against ethnic Albanians, and targeting them in the hills and forests of the largely rural province has proven difficult.

Two weeks into its air campaign against Yugoslavia, NATO was mulling strategy. Britain's domestic news agency Press Association said Defense Secretary William Cohen would confer Thursday at Ramstein Air Base, Germany, with his counterparts from Britain, France and Germany about airstrike tactics.

Foreign ministers from NATO's 19 member nations were to meet Monday at alliance headquarters in Brussels, Belgium, to discuss the deteriorating situation in Kosovo, the first meeting of NATO foreign ministers since their regular winter session in December.

On Wednesday, Albania's parliament approved NATO plans to deploy 24 U.S. Apache attack helicopters _ a move that should bolster the alliance's firepower against the Yugoslav tanks and armor.

The allies have vowed to keep up the pressure on Yugoslav President Slobodan Milosevic until he pulls back his forces from Kosovo and allows ethnic Albanian refugees to return under the protection of an international security force.

Before the airstrikes, ethnic Albanians made up 90 percent of Kosovo's 2 million people.

Meanwhile, Spyros Kyprianou, the speaker of the Cypriot Parliament who has had a close relationship with Yugoslav leaders, was in Athens, Greece, today hoping to fly to Belgrade to make arrangements for the release of three American soldiers captured by Serb forces on March 31.

Yugoslavia expects NATO bombing to ease if it turns over the Americans, Kyprianou said today. He also said the most recent airstrikes have given Yugoslav officials second thoughts about turning over the servicemen.

``It is only too evident that Yugoslavia has been upset by last night's heavy bombing,'' Kyprianou said. ``They now expect some parallel relaxation.''

State Department spokesman James P. Rubin has said the United States is insisting the soldiers be released unconditionally because they ``shouldn't have been held in the first place.''

Still, Kyprianou said, the Americans could be freed during the long Orthodox Easter weekend, which began today.

``While the Americans could be released over the Easter weekend to join their families, NATO could reciprocate by responding to the numerous international appeals for a cease-fire, at least over the Orthodox Easter weekend,'' Kyprianou said.

Easter is celebrated Sunday throughout the Orthodox world, including in Serbia and among Greek Cypriots.

Despite the troubles, Kyprianou said he remained ``very hopeful'' he would be able secure the Americans' release following contacts Wednesday night and early today with Greek, Russian and Yugoslav diplomats.

Yugoslavia's ambassador to Mexico, Dusan Vasic, said he expected the three soldiers to be freed ``as a gesture of international goodwill.''

Any release could be a sign that Milosevic was seeking a way out of the confrontation with NATO, which began punishing airstrikes March 24 to force him to accept a Western peace deal to end fighting in Kosovo.

Yugoslavia's leadership announced a unilateral cease-fire with the rebel Kosovo Liberation Army effective Tuesday night in advance of this weekend's Orthodox Easter.

But Western leaders vowed to continue the air campaign until Milosevic withdraws special police units from Kosovo and accepts the peace plan, which includes 28,000 NATO-led troops to enforce it.

``Milosevic is fast approaching a point where getting some kind of negotiations are in his interest in order to halt the bombings and to forestall the decision to introduce ground forces,'' said Ivo Daalder, a former Clinton administration official now with the Brookings Institution in Washington.

In addition to strikes on Belgrade and Kosovo, Serb media said several industrial and garrison towns were targeted in the latest wave of attacks by NATO missiles and warplanes.

Six missiles struck the provincial town of Cuprija, 55 miles south of Belgrade, early today, Studio B television reported. Yugoslav media also reported explosions late Wednesday in Pancevo, an industrial town just north of Belgrade that has been repeatedly targeted during the 15-day NATO campaign.

Also hit was the industrial town Kraljevo, 75 miles south of Belgrade on a road leading toward Kosovo, Serb TV said. The missiles struck ``mostly on civilian targets,'' the report said. No other details were available.

In the northwestern town of Sombor, near the Hungarian border, a raging fire was shown on Serb TV, but the report did not specify what was hit. The town, which has a large garrison and a military airport, has been targeted for three days in a row.

It was not possible to independently confirm the reports.

The latest air activity followed a move by Yugoslav authorities Wednesday to seal off the borders with Macedonia and Albania after forcing more than 450,000 refugees out of Kosovo.

Thousands of refugees who had been waiting on the Yugoslav frontier were turned back to villages or towns said to have been ravaged amid the campaign of forced expulsions of the ethnic Albanians.

``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.

Along the Albanian border, Yugoslav guards could be seen laying what appeared to be mines and digging fortifications just inside their territory.

In Brussels, NATO spokesman Jamie Shea said NATO was ``deeply preoccupied'' by reports of atrocities against civilians and the burning of 50 villages in Kosovo since Saturday. He said three mass gravesites had been identified, based on refugee testimony.

The KLA claimed Serb forces killed at least 51 civilians, including 12 children, during a raid on the village of Celina, southwest of Pristina. A statement by the rebel Kosova Press listed names and ages for all the alleged victims.