The Prime Minister used question time to stress his "deep regret" at the bombing of a residential area in the southern Yugoslavian town of Surdulica where, Serb officials say, at least 17 people died.
But he made clear that allied forces were taking "every single precaution we possibly and reasonably can" to prevent civilian injuries during the bombing raids. The difference between Nato and President Slobodan Milosevic is that "whenever there are civilian casualties as a result of allied bombs they are by error, we deeply regret them when they come, but they weren't the only tale of casualties overnight.
"There were also the 200 Kosovo Albanian men that were massacred ... adding to the earlier reports of some 470 men killed previously. These people the Serb paramilitaries are killing are killed deliberately and that is the difference between us and them."
To cheers, he added: "We must carry on with this action, utterly united, utterly resolved to see it through to a successful conclusion."
But Mr Blair faced fresh criticism from Labour backbenchers when Tony Benn, the MP for Chesterfield, accused Nato of committing "war crimes". He told the Prime Minister: "Are you aware that although the whole House and the world are united in horror and opposition to the ethnic cleansing and brutality by Milosevic ... the killing of innocent civilians ... does nothing in the short run to help the refugees, and in the views of many people, including myself, amount to war crimes themselves."
But Mr Blair insisted: "We either acted or we stood aside and let this continue ... I think that would be the ultimate moral outrage!"
Harry Barnes, the Labour MP for Derbyshire North East, challenged the Prime Minister on whether Nato was bombing the right targets. "In bombing Yugoslavia, what is the distinction between bombing acceptable military targets and unacceptable civilian targets?
"And why are factories, bridges and TV stations included within the first category rather than in the second? Surely there should be a distinction between the two?"
But Mr Blair stressed the bridges had been used "to transport Serb militia and troops. They are a vital part of Milosevic's war machine."
William Hague renewed Tory support for the air strikes,saying: "President Milosevic need not think that these tragic consequences will weaken our resolve in this House that the Nato action will succeed."Reuse content