In response to warnings from Russia that it would ignore Nato's attempts to impose an embargo, Nato Secretary-General Javier Solana said contacts with Moscow would be "intensified".

However he said the alliance was determined to do "whatever is in our means and our capabilities" to stop oil reaching Serb forces.

According to the UK Foreign Secretary, Robin Cook, both Russian and Nato naval forces will "take good care" that a confrontation does not happen.

"Any Russian flow is likely to go down the Black Sea and through the Danube, " said Mr Cook on the BBC's Breakfast with Front programme.

Katty Kay: Nato has not yet agreed the mechanics of how to impose the embargo

As Nato set out its position, its warplanes again bombed Yugoslavia.

Serb authorities say 50 missiles hit Pristina and the surrounding area in a six-hour campaign of intense sustained bombardment.

Mike Williams in Belgrade: "For six hours last night, targets in and around Pristina were hit"

Yugoslav state television went off air again, after Nato bombs reportedly destroyed an electrical transformer station at Mount Avala, to the south of Belgrade.

There have also been other unconfirmed reports of more deaths of Albanians in Kosovo; local sources said five children died and six were injured when a group of them tending cattle disturbed an unexploded cluster bomb in the countryside south of Orosovac.

Elsewhere four missiles were reported to have hit an industrial zone in Nis, 120km south of Belgrade, early on Sunday morning causing "great damage", but there were no reports of casualties.

The local media also reported smoke rising from an unspecified target in the centre of the city.

Five missiles were also reported to have hit a chemical plant in central Serbia in Lucani early on Sunday.

'Visit and search'

US President Bill Clinton has defended Nato's planned sea searches, saying it was unreasonable to ask pilots to risk their lives attacking oil depots when Serbia could get fuel from ships.

Nato leaders are in effect attending a "war summit"

Nato is drawing up plans to "visit and search" ships to try to prevent supplies reaching Serbian armed forces via ports in Montenegro.

Urging support for the embargo, French President Jacques Chirac said Nato had to be "very cautious" as stopping ships at sea could be seen as an act of war.

Russian Foreign Minister Igor Ivanov said his country would continue to supply President Slobodan Milosevic's government with fuel.

Analyst Professor Margot Light: Russia will not want to lose face

"We cannot do anything to worsen the suffering of the people in Yugoslavia, and we will continue delivering oil in keeping with our international commitments," Mr Ivanov said.

Russian special envoy Viktor Chernomyrdin, who returned from Belgrade with a fresh peace offer on Thursday, backed up Mr Ivanov.

"The blockade (idea) is bad. The strikes are bad. When people die, it's bad."

Nato has said that if it can "switch off the oil tap completely" to Yugoslavia, the Yugoslav military machine will grind to a halt quickly.

Yugoslav Deputy Prime Minister Vuk Draskovic said the action would hit civilians.

"Nato is punishing those who are innocent. It means punishing poor Serbian civilians and forcing them to drive no car ... That's against people."

(Click here to see a map of Friday night's Nato strikes)

March for Serbian TV dead

People opposed to the bombing of Yugoslavia have taken to the streets of the US, Italy and Germany.

Jonathan Marcus: Kosovo will decide Nato's future role

Outside the White House in Washington, more than 2,000 demonstrators, many of them Serbian-Americans, waved Serbian flags, called for an end to the war and released black balloons.

Italian police said tens of thousands of people protested in several main cities, including Naples, the headquarters of Nato's southern command.

German peace campaigners said tens of thousands of people demonstrated in 40 cities against the Nato action and the expulsion and persecution of people in Kosovo.

More than 100 peace activists travelled from Germany to Belgrade where they lit candles and placed flowers in front of the state television building that was bombed on Thursday night. Thousands of Belgrade residents also marched in protest at the bombing.

The Yugoslav authorities have said the number of deaths from the bombing has risen to 20. Rescue workers were continuing to search the rubble, but they said signals from trapped people hitting on pipes and pieces of masonry had stopped.

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