Reports from Serbia said the latest air raids had caused blackouts in a number of cities, including Nis, Novi Sad and the capital, Belgrade.

The official Yugoslav media blamed the use of graphite bombs which scatter thin strands of highly conductive material short-circuiting equipment.

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

Nato says Thursday saw the heaviest raids it has carried out so far, hitting military targets in Kosovo.

In another development, the White House said President Clinton would phone his Chinese counterpart Jiang Zemin - a sign that tensions over Nato's bombing of the Chinese embassy in Belgrade were easing.

BBC's Mike Williams in Belgrade: "Powercuts convince people they're the targets"

In Washington, Air Force General Charles Wald told journalists that the Apache AH-64s "are ready to go right now."

Wald said they could be used as soon as the order was given by President Clinton and Nato's supreme commander, General Wesley Clark.

Reports said that the Apaches would be conducting a live-firing exercise in Albania near the Kosovo border prior to any combat missions.

BBC's Stephen Gibbs analyses the most recent developments

The deployment of the Apaches suffered long delays, and two of them were lost in training accidents.

Kosovo 'a moral issue'

On Thursday, President Clinton stressed his determination to defeat the Yugoslav President, Slobodan Milosevic, saying the Kosovo crisis was a moral and strategic issue for the US.

President Clinton: Kosovo is a moral and strategic issue

Mr Clinton insisted that the bombing would continue as long as it took to convince the Yugoslav leader to agree to Nato's demands.

"The Kosovars must be able to return home and live in safety," he said.

In his speech, Mr Clinton went on to justify the Nato campaign, giving details of the atrocities carried out by Serb forces against Kosovo Albanians.

He compared President Milosevic's ethnic cleansing with the Holocaust.

China tensions easing

China has started to repair the damage to relations caused by Nato's bombing of the Chinese embassy in Belgrade a week ago.

Duncan Hewitt in Beijing: "Clinton's condolence may go some way to appease Chinese anger"

The White House announced that President Clinton was to speak on the phone to his Chinese counterpart, Jiang Zemin.

The announcement followed a brief meeting between Mr Clinton and the Chinese Ambassador in Washington, Li Zhaoxing, during which the US President signed a condolence book for the victims of Nato's bombing of the Chinese embassy.

Earlier in the week, the Chinese President had refused to take a call from President Clinton.

UN mission planned

The United Nations is planning to send a humanitarian mission to Yugoslavia on Saturday, for the first time since the airstrikes began.

Fifteen representatives are to be escorted by Yugoslav police, on what the UN says will be a 12-day exploratory tour of the country which will include visits to Kosovo.

The mission's leader, the Undersecretary-General for Humanitarian Affairs, Sergio Vieira de Mello, said they had been assured of unhindered access.

On Thursday, President Milosevic refused to see the UN Human Rights Commissioner Mary Robinson.

During a visit to Belgrade, she said Yugoslavia had to commit itself to the unconditional and safe return of all refugees.

She said she regretted that President Milosevic had refused to meet her.

But she tempered her criticism by saying the Nato strikes had brought cruel suffering to innocent civilians in Yugoslavia.

Raids stepped up

Nato was given a boost by news from Germany, where the Green Party voted against a pacifist motion and instead supported the country's continued involvement in the Nato attacks on Yugoslavia.

Bridget Kendall reports on growing opposition to Nato action

Nato has also been strengthened by the US's announcement of the deployment of an additional 35 aircraft.

The alliance has kept up the pressure on Serb forces in Kosovo by mounting raids during daylight hours.

• Refugees recount fresh atrocities
• Greens back leader over Nato
• Analysis: Sacking could boost Kosovo peace
• Bulgaria says Nato bombing will succeed
• Macedonia complains over Nato aid

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