Mr Blair made the pledge in a 35-minute telephone conversation with US President Bill Clinton.

And speaking on BBC One's Breakfast With Frost, UK Defence Secretary George Robertson said: "There may well be a case of sending more fighter bombers out there, and I'll be making an announcement in due course about it."

The RAF currently has eight GR7 Harrier jets from RAF Wittering in Cambridgeshire stationed at the Gioia del Colle airbase in southern Italy.

Andy Tighe reports: "A tragic sorting out of stories"

A Downing Street spokesman was unable to say whether the RAF would be sending more planes in light of Mr Blair's pledge.

'Defending the defenceless'

Mr Blair told the BBC that Nato was forced to act to protect the ethnic Albanian population of Kosovo.

He said: "For these poor, defenceless people we are the only chance they have got.

Tony Blair: "We have to see this through"

"Milosevic is carrying on with the repression but he also now knows that he is having to suffer the consequence of Nato air strikes."

The move to Phase II of Operation Allied Force follows indications on Saturday that strikes on more than 40 targets across Yugoslavia had disabled Yugoslavia's air defence system.

Tony Blair: "We are the Kosovars' only hope"

The Downing Street spokesman said the two leaders reviewed the situation and agreed to intensify the Nato campaign in light of continuing Serb atrocities against ethnic Albanians in Kosovo.

Consulted with Solana

The offer of increased UK air power was made following consultations with Nato Secretary General Xavier Solana.

The BBC's Defence Correspondent Mark Laity: "Nato is now looking at moving targets"

Mr Clinton and Mr Blair have agreed there can be no let-up in the air strikes while the killing continues on the ground.

The two leaders apparently also discussed how to keep tabs on individual Serb commanders in the field so they could be held responsible for war crimes.

A British soldier trains with a Challenger battle tank near Skopje, Macedonia

Mr Solana said the decision to step up to Phase II was designed to "bring a halt to violence in Kosovo and prevent further humanitarian catastrophe".

He said: "With this aim in mind, the broader range of operations will allow Nato commanders to intensify their action against Yugoslav forces."

As he spoke Nato launched fresh strikes against Serb forces, with reports of an explosion in the centre of Belgrade.