The admission came as the alliance said it had carried out its most intensive bombing of Yugoslavia since the air campaign began two months ago.

After days of weather problems, the alliance said it had launched 684 sorties in the latest 24-hour period, hitting a wide range of targets across Yugoslavia.

But among the targets hit was a stronghold of the KLA at Kosare near the border with Albania because Nato thought the position was still held by the Yugoslav army.

International monitors said one KLA guerrilla was killed and at least 15 others were wounded.

Prison strike defended

Istok: "A sizeable military conplex"

On the subject of the controversial attack on the prison in Istok, the alliance said the prison was just a small part of what is a major military facility which has been used for a long time by the Yugoslav army and special police.

"You only have to look at it," said Nato spokesman Jamie Shea showing a photograph of the area. "Airstrip, helicopter pad, military barracks and installations.

"The prison, or rather the detention and interrogation centre, is only one aspect of this. It was a sizeable military compound.

Targeting debate

There has been much debate recently about Nato's targeting policy. But its latest operations were against a broad range of targets; fuel depots, roads, bridges, air defence sites, television repeater aeriels and military communications.

The BBC's Nick Childs at Nato headquarters: "The highest number of sorties since the air campaign began "

Nato says its forces hit over 30 elements of Serb heavy forces in Kosovo, including at least 12 tanks.

It also confirmed it had mounted extensive attacks on Yugoslavia's electricity generation system.

Parts of Belgrade and other towns and cities across the country were still without electricity hours after Nato warplanes attacked half a dozen key transformers on the Yugoslav power grid.

Limited supplies have been restored in some areas. The power company has been shunting electricity from one neighbourhood to another throughout the day so that food can be cooked and water can be pumped, for a few hours at least.

Ground troops pressure

As the air campaign intensifies again, the Pentagon has called for the urgent deployment of ground troops for a peace implementation force.

Details of how a force might be deployed and in what circumstances are continuing to be discussed between Russian and Western diplomatic envoys at a series of meetings.

BBC Diplomatic Correspondent Kevin Connolly: "A huge task ahead"

But the Pentagon also wants Nato to be ready to look at other options if the air campaign does not produce the desired results in time.

Nato has confirmed that planning for a variety of options remains under review. But its priority, it says, is the operational plan for the peace force, which is now with Nato ambassadors and will be discussed next week.

More refugees

Thousands more refugees cross into Albania

After a lull, thousands more refugees have escaped from Kosovo. Nearly 3,000 crossed the border into Albania overnight, the largest number there for several day.

But it could be a long time before they are able to go back home. The head of the Nato humanitarian force in Albania warned that it could take up to two years to return all the refugees to Kosovo.

Lieutenant-General John Reith said it would take that time to repair the devastation caused by Serb forces.

There is also growing concern for displaced persons within Kosovo.

The political leader of the Kosovo Liberation Army, Hashim Thaci, said that at least 600,000 people were currently homeless in Kosovo and not receiving any humantarian aid from the international community.

• Analysis: The shifting diplomatic scene
• Nato: Prison bombing 'justified'
• Analysis: The war for hearts and minds
• Anti-Yugoslav protest in Montenegro
• Children's tales of horror

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