The Serbs say the raids in the town of Istok killed up to 19 people.

Alliance spokesman Jamie Shea said the prison was first and foremost a military target, as it was being used as a barracks for the Yugoslav Army and special police.

Meanwhile, the United States has indicated a shift in its stance on ground troops.

Pentagon spokesman Ken Bacon said Nato must consider other military options if the air campaign had not worked by the autumn.

The BBC's Paul Reynolds: "US beginning to shift, says Britain"

He also called for a rapid build-up of Nato troops in Macedonia, to help refugees return to their homes in Kosovo.

Nato's political body, the North Atlantic Council, is likely to discuss a plan early next week for a peace-keeping force of about 50,000 troops, he said.

There was also talk across Washington about the force possibly going into Kosovo in a non-permissive environment - something the Americans have refused to contemplate before.

Prison bombed twice

The BBC's Bridget Kendall: There are reports of a new rift between the US and Britain

The prison bombed by Nato held a number of rebels from the Kosovo Liberation Army as well as criminals, according to the Serb authorities. They said 15 missiles hit the complex, severely damaging a number of buildings.

It was the second time in three days that Nato attacked the prison and, as visiting journalists left the complex, another raid began.

Nato insists the bombing was not an intelligence blunder. Mr Shea said that if prisoners had been in the jail as well, that was the responsibility of the Serbs.

Show of unity

UK Foreign Secretary Robin Cook visited the US for a meeting with US Secretary of State, Madeleine Albright, designed to show unity.

The BBC's Bridget Kendall: There are reports of a new rift between the US and Britain

During their meeting Mrs Albright and Mr Cook told the BBC that both governments were determined to maintain the bombing and were not, at present, considering an Italian proposal for an early suspension of the air campaign.

Mr Cook said Nato bombs were hitting so hard that "I don't see any sign that the Yugoslav Army at the present rate of attrition is going to hold out until August or September".

German Foreign Minister Joschka Fischer said at a meeting with the Swiss foreign minister in Bonn there was an urgent need to discuss Nato's bombing strategy, including damage caused to the Swedish, Spanish and Swiss embassies.

Washington has apologised to the Swiss Government for the strike on its embassy, which damaged the building but caused no casualties.

But alliance officials insist that there has been no request from its members for a change in campaign strategy or targeting policy.

(Click here to see a map of the most recent Nato strikes)

In a separate development, Kosova-Albanian rebel leader Hashim Thaci has revealed he has asked moderate Kosovan leader Ibrahim Rugova to meet him urgently in Tirana.

Mr Thaci also called for Mr Rugova and his followers to take their seats in the government as laid out in the Rambouillet peace accord.

Kosova Albanians have up to now been split between those loyal to the rebel Kosovo Liberation Army and the pacifist policies of Mr Rugova.

Diplomatic marathon

In Moscow, marathon talks between the Russian special envoy, Viktor Chernomyrdin, and his US and European counterparts broke up with no clear sign of progress.

The American Deputy Secretary of State, Strobe Talbott, said he and Finnish President Martti Ahtisaari would return to Moscow next week for further negotiations.

However, the process hit a snag when Mr Chernomyrdin abruptly, and for no apparent reason, cancelled meetings on the issue with Greek Foreign Minister George Papandreou and UN envoy Carl Bildt.

The Russian Foreign Minister, Igor Ivanov, said Moscow and the West were still at odds over the composition of an international force for Kosovo. He repeated Russia's demand for an immediate halt to Nato's bombing campaign.