"We take every single precaution we possibly can to prevent civilian casualties," he told MPs at Prime Minister's Questions.

On the same night Nato pilots hit a residential area of a Serb town, killing about 20 people, massacres of Kosovan men continued, he said.

The prime minister referred to reports of 300 Kosovo Albanians being slaughtered on Tuesday night near the village of Djakovica.

Serbians hold a candlight vigil in Belgrade

"Whenever there are civilian casualties as the result of allied bombs, they are errors," he said.

"These people the Serb paramilitaries are killing are killed deliberately - that is the difference between us and them."

It would be the "ultimate moral outrage" to do nothing in the face of on-going atrocities.

Hague questions ground plans

Conservative leader William Hague quizzed the prime minister on plans for sending in ground troops to Kosovo.

Government spokesmen had suggested a ground force might enter Kosovo in about a month, he said. But to achieve Nato's aims and allow the refugees to return before winter, troops would be needed sooner.

Mr Blair said: "It is certainly right that time is a factor in this." This had led Nato to take the decision to review its options at its 50th anniversary summit.

The effectiveness of the oil blockade was also questioned by Mr Hague.

William Hague: "The rules of engagement must be absolutely clear"

It would be "as effective as we can possibly make it, consistent with proper rules of engagement and international law", the prime minister replied.

Contrary to some reports, the United States had implement a trade blockage, including oil, on Yugoslavia.

Benn's 'war crime' charge

Former Labour Cabinet minister Tony Benn made the strongest attack on the United Kingdom's involvement in the Balkan crisis.

A column of smoke after a bombing in Montenegro

He described the bombing of Serbia, including the recent attack bringing civilian deaths, as a "war crime in itself".

The prime minister said: "I simply ask him, in those circumstance what does Nato do?

Tony Benn: "Nothing in the long run to help the refugees"

"We either acted or we stood aside and let this continue."

Fresh tales of atrocities continued to emerge from Kosovo daily, Mr Blair said.

"We have to recognise the fact that the only chance these refugees will ever get is if Nato continues its actions and is successful.

"The best and most eloquent advocates for the Nato action are the refugees themselves."