The UK prime minister, in a statement to Parliament on the Alliance's 50th anniversary summit in Washington, said member states shared a totally unified commitment to end "ethnic cleansing".

The BBC's Bridget Kendall: "Nato air strikes have wiped out a vital military supply route"

Apart from stepping up the air campaign, Nato is examining ways to prevent Yugoslav President Slobodan Milosevic importing fuel to replace supplies already destroyed by the bombing, he said.

On the possibility of the deployment of ground troops, Mr Blair said it had been right that Nato had decided to update its plans for the use of ground troops.

"Meanwhile the build-up of forces in the region continues," he said.

Tory leader William Hague said his party would continue to support the alliance's action over Kosovo.

William Hague: Government must clarify ground troops policy

"It is vital that Nato finishes what it has started," he said.

But he added that the government's apparent shift in policy towards the use of ground troops needed a "fuller explanation than has so far been given".

Mr Hague asked whether the use of ground forces was now the UK's preferred strategy.

Tony Blair: "A total commitment by all members of the alliance to defeat and reverse the policy of ethnic cleansing"

Mr Blair replied: "I do think there is a limit to the degree with which we should discuss every possibility of strategy.

"We continue the air campaign. We do not rule out any options."

'Evil beyond belief'

Mr Blair said emerging accounts of Serb atrocities in Kosovo revealed "evil beyond belief".

Fleeing refugees told of women being raped in front of their families and men forced to dig their own graves before being shot.

The United High Commission for Refugees had also confirmed that civilians had been used as human shields to prevent Nato attacking a military target, he said.

"The acts remain the essential justification for Nato's actions," Mr Blair said.

"We cannot allow the policy of ethnic genocide to go unchecked."

Mr Blair hailed Nato's approval of a security pact with frontline states regarded as being at risk of attack from President Milosevic's forces.

The pact will offer military assistance: to Romania, Slovenia, Macedonia, Bulgaria, Albania, Croatia and Bosnia.

Mr Blair again stressed it was not an aim of the Nato action to remove Mr Milosevic from power.

But he added: "While Milosevic remains, the security of the region is more difficult."

Robertson to visit Balkans

George Robertson: Milosevic will not prevail

Earlier, Defence Secretary George Robertson stressed that Nato had no intention of making a wholesale organised invasion of Kosovo against any substantial Serb opposition.

He told a Ministry of Defence briefing that he that he would visit Macedonia and Albania later this week for talks.

Mr Robertson praised Macedonia for dealing with the flood of refugees from Kosovo and for allowing Nato troops to mass on its borders.