The Harrier GR7 ground attack jets took off from the Gioia del Colle base in Italy as up to 100 warplanes took part in the latest attack.

Nato officials were quoted as saying all allied aircraft had returned safely to their bases following Thursday night's action.

Mark Laity reports from Nato Headquarters: "All the planes have returned"

British military sources said the Harriers had taken part in a "successful" mission against Serb targets.

More Tomahawk Cruise missiles fired from warships in the Adriatic were also involved, with reports suggesting a much greater number than in the first wave of bombing.

The focus of the RAF attacks is expected to change from air defence systems to increased bombing of Serbian military units on the ground.

A BBC correspondent at the Italian base, Jonathan Charles, said the Harrier crews were more confident of a successful bombing mission following aborted raids on Wednesday night.

He was prevented from saying how many British jets were involved but believed it was "just the first wave of what is going to be a multi-wave attack" on targets in Serbia and Kosovo.

The fresh strikes were confirmed by US defence spokesman Ken Bacon.

Jonathan Charles in Italy: "This will be a multi-wave attack"

"It will be another substantive strike. It will be severe," he said at the Pentagon.

Prime Minister Tony Blair promised that attacks will continue while Yugoslavia remains defiant over Kosovo.

Nato sources in Brussels are warning that their job has only just begun and the attacks may go on for days.

PM to make TV broadcast

Mr Blair plans to make a nationwide television broadcast on Friday evening to argue the case for military action.

Labour peer Lord Jenkins of Putney has resigned the party whip in the House of Lords in protest as the action which he said was "criminal barbarity".

Joe Paley reports as the Harriers leave the Italian base

Yugoslavia, for its part, has severed diplomatic relations with Britain.

But as the planes took off, Foreign Office minister Tony Lloyd said that Nato's resolve to achieve its stated objectives was "absolute".

"I don't believe that there's any doubt amongst all the Nato states that we have to continue this action until we have achieved the objectives we have set out for ourselves," he said.

Mr Lloyd said the responsibility for bringing an end to the attacks lay squarely with the Yugoslav president.

"We all want an end to this military conflict, but it's only Milosevic who can take the decisions that will put it to an end."

Why pilots turned back

Joe Paley in Italy on how jets and pilots are prepared for their 'lonely and dangerous' journeys

Before the renewed strikes began, RAF pilots at the Italian base revealed why they suddenly halted Wednesday's first bombing mission.

Six Harriers took part in the first round of air strikes, but only one dropped a bomb, which fell short of its target. The others turned back to the base with their bombs still on board.

Jonathan Charles in Italy: One pilot said he was 'scared stiff'

One of the pilots, who did not wish to be named, said the mission was aborted to avoid unnecessary casualties.

"If we can't hit the target we do not want to start slinging our bombs around and creating collateral damage," he said.

George Robertson: "We hit hard"

Earlier, Defence Secretary George Robertson said poor visibility was preventing the pilots from seeing their targets.