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The Serbian and federal interior ministry buildings which were hit are close to a major hospital complex and several apartment blocks.

The latest assault came as tens of thousands of refugees continued to pour out of Kosovo. Nato says more than a third of the population has fled the province since the start of the conflict last year.

Buildings were still smouldering at daybreak

A series of loud explosions rocked the capital around 1 a.m. local time (2300 GMT), about 20 minutes after air raid sirens wailed over the city.

Within minutes scores of ambulances had rushed to the scene, but a BBC correspondent in the city quoted doctors saying the only casualties were firemen and policemen hit by falling glass.

John Simpson in Belgrade: Bombing with 'extraordinary precision'

The ministries are situated on a street known as Embassy Row with the US, British, German and Canadian embassies all close by.

Nato says they were targeted because they control the security forces accused of committing atrocities against Kosovo's Albanian population.

John Simpson in Belgrade: "The two ministry buildings were taken out in their entirety"

"Downtown Belgrade is on fire on the biggest Christian day of Easter,'' he told the British satellite station Sky. ''The last time Belgrade was on fire [over] Easter was in 1941 when Hitler bombed it."

Russian condemnation

Russian State Duma Deputy Speaker Sergey Baburin, who is in Belgrade, said the latest Nato raid showed ''a cannibal'' had appeared in Europe.

"This cannibal must be stopped and the Russian president, parliament and government are already taking steps to do that," added Mr Baburin after seeing the gutted buildings.

But a Pentagon spokesman said: "There should be no surprise about what has happened in Belgrade. We have said there is no sanctuary and this action is consistent with a continued and sustained policy."

Air raid sirens also sounded in Novi Sad, Nis, Kragujevac and Bor and there were reports of attacks north west of Belgrade.

Refugee crisis

Aid agencies are warning of a humanitarian disaster in Yugoslavia's neighbours not seen in Europe since the Second World War.

The United Nations refugee agency, the UNHCR, says the situation is reaching nightmare proportions. It estimates more than 180,000 people have crossed into Macedonia, Albania and Montenegro in the last 10 days.

But the Yugoslav Government blamed the exodus on the Nato bombings and appealed to Kosovans not to leave.

In a statement issued by the Yugoslav mission to the United Nations, Belgrade said only terrorists were not wanted in Yugoslavia.

A woman is carried to the border in a wheelbarrow

Mr Clinton is sending his Deputy Secretary of State, Strobe Talbott, to the region where he will try to persuade other countries to accept refugees.

Western officials say they also have evidence that Mr Milosevic is plotting to oust the democratically-elected president of Montenegro, which - together with Serbia - forms Yugoslavia.

Milosevic remains defiant

Meanwhile, the Serbian authorities have begun collecting evidence for proceedings against three US soldiers captured by the Yugoslav army on the border between Macedonia and Kosovo.

Nicholas Witchell reports: ''Critical days for Nato''

Mr Milosevic has asked Moscow for military aid during a meeting in Belgrade with Russian MPs headed by Mr Baburin,

So far Russia has ruled out giving military support. But President Milosevic said: "We will defend our country,'' he said. ''If we have assistance, then we will defend it more easily, and if we have none, then defending it will be harder, but we will definitely defend it."

The US told Russia it would be a grave mistake to give Yugoslavia any sort of military assistance against Nato.

The warning came as a Russian reconnaissance ship made its way to the Mediterranean to monitor events in the Balkans. Moscow says the vessel will stay out of the conflict and not share intelligence with Belgrade.

A US Navy aircraft carrier is also approaching the Mediterranean. Defence officials said USS Theodore Roosevelt was likely to be deployed in the conflict.