Speaking on Saturday after raids on army and police buildings in central Belgrade, Mr Blair said attacks were now being targeted at Serbia's "killing machine".

The government later pledged another £10m in emergency aid for Kosovo's refugees and proposed the setting up of a Nato-protected sanctuary for them in Macedonia.

Tony Blair said the attack on Belgrade was the start of the next phase of the air strikes.

He said: "The whole essence is to keep up this pressure, to intensify it, to carry it on with total resolve.

"There is no way no we can stop until the terms Nato has set out are fully complied with."

Tony Blair: "We won't let you down"

In a message to Mr Milosevic, he said: "Your policy of ethnic cleansing is an act of barbarity. We will defeat you over it."

He promised that Serbia's gains in Kosovo would be reversed - promising Albanian refugees: "We will not let you down."

Friday night's Cruise missile attack on Belgrade was "highly successful" in damaging and disrupting the Serb war machine, including the command and control centre and forces on the ground, he said.

Tony Blair: Strikes will intensify

Fires engulfing two government buildings in the heart of the Yugoslav capital raged through the night.

Mr Blair spoke by telephone to several Nato leaders, including a 20-minute conversation US President Bill Clinton.

Later Foreign Secretary Robin Cook announced a new proposal for refugee "sanctuaries" in Macedonia, where 50,000 people are waiting in freezing conditions at the border.

Mr Cook said the sanctuary would be created, protected and supplied by Nato troops and jointly run by Nato and the UNHCR.

Refugee crisis worsens

Food, blankets and medicines would be supplied by a Nato "air bridge" into Macedonia.

Fellow Nato member Germany said it would accept some refugees - and talk to other EU countries about also taking them in.

International Development Secretary Clare Short will fly to the area on Sunday with a team of officials to discuss how the UK can help.

The Balkan refugee crisis was increasing by the day, Army Major Julian Moir said in a briefing on Saturday.

He said: "It got very much more serious over the past 24 hours."

Meanwhile, Air Marshal Sir John Day said the raids were increasing in "intensity and scope".

He said that although attacks by aircraft were hampered by bad weather, there had been strikes - including Tomahawk missiles fired by Royal Navy submarine HMS Splendid.

On Saturday, eight B52 bombers took off once more from RAF Fairford in Gloucestershire.

A US Air Force spokesman said they were fully loaded with missiles, but would not comment on whether the aircraft were on a combat mission or a training flight.

In London, UK-based Albanians held a rally to demonstrate support for the Nato action and call for ground troops to be sent in.