More than 60 refugees in are reported to have died in an attack on Wednesday in southern Kosovo.

Nato has confirmed its preliminary investigations show a bomb dropped from an allied aircraft hit a civilian vehicle travelling in a convoy.

Tony Blair contrasts Nato concerns for civilians with Serbian aggression

"Serb police or military vehicles may have been in or around the convoy," it said in a statement. "Nato has no way of knowing the number of casualties caused by the incident."

The foreign secretary said RAF planes had not taken part in strikes against moving targets on Wednesday.

But he poured scorn on criticism of Nato from the Serbian military.

"If Nato planes were responsible for civilian loss of life, that is something that will cause us deep concern," the foreign secretary said.

"But I have to say that I will not accept the criticism that has been emanating loudly from Belgrade, from the people who organised the mass ethnic cleansing of Kosovo, who have caused thousands of civilian deaths in Kosovo."

Nicholas Witchell: "A matter of great concern if Nato is found to be responsible"

These actions resulted "not through any miscalculation or misjudgement, but by a deliberate programme of intent", Mr Cook said.

"How dare they now produce crocodile tears for people who killed in the conflict for which they themselves are responsible."

A boy at the scene of the bombing

Mr Cook said Serb President Slobodan Milosevic had to answer a series of questions about the refugees, rather than trying to gain public support from their deaths.

The outside world wanted to know why special police vehicles were escorting a column of refugees and where they were taking them.

The foreign secretary's view was echoed by Prime Minister Tony Blair.

"Of course we regret these things deeply when they happen," he said.

"But that should not make us flinch from placing responsibility for this conflict squarely on the shoulders of the person, Milosevic, who has begun this conflict."

Mr Blair's official spokesman also laid the blame clearly with President Milosevic.

"The prime minister has got a very, very clear view that every single act of suffering in Kosovo has been caused by Milosevic.

"The idea that suddenly Milosevic is this caring person who's worried about the fate of Kosovar Albanian refugees, I think most people would find revolting.

"He's been killing, shelling, raping, torturing," the spokesman said.

Both the UK Government and other Nato members have admitted civilian casualties are inevitable in any conflict, although the allies stress they go to extreme lengths to avoid them.

But Downing Street equally warned the truth about the refugee deaths had not been fully established, while Serbia had lied in the past.

"Regardless of what the full facts are, you simply can't believe what the Serbs say," the spokesman said.