In an interview on Friday on Al-Jazeera's new English-language channel, broadcaster Sir David Frost suggested that the 2003 US-led and British-backed invasion had "so far been pretty much of a disaster".
"It has," Blair replied, before adding quickly: "But you see, what I say to people is why is it difficult in Iraq? It's not difficult because of some accident in planning.
"It's difficult because there's a deliberate strategy... to create a situation in which the will of the majority for peace is displaced by the will of the minority for war."
But during Blair's trip to Pakistan for talks with President Pervez Musharraf, the prime minister's official spokesman told reporters: "It was a straightforward slip of the tongue... sometimes he does this when he's half-listening to the question and wants to get on and respond."
The spokesman insisted that Blair did not think Iraq was a disaster.
"But what he does acknowledge is that there are difficulties and he doesn't in any way try to downplay those difficulties," he added.
Earlier, another Downing Street spokesman told AFP that Blair "does not use the word disaster".
Responding to the comments, Sir Menzies Campbell, leader of the Liberal Democrats, Britain's third-biggest political party, lambasted the government over its record in Iraq and demanded that Blair say sorry.
"If the prime minister accepts that it is a 'disaster' then surely parliament and the British people, who were given a flawed prospectus, are entitled to an apology," he said.
A spokesperson for the main opposition Conservatives added that the remarks highlighted the need for an inquiry into how Britain joined the war in Iraq.
During the interview, Blair also urged Syria and Iran to become partners in the search for peace in the Middle East -- or face isolation on the world stage.
His comments were broadcast on the same day as it was reported that one of his most loyal ministers had branded Iraq his "biggest mistake in foreign affairs".
Industry minister Margaret Hodge also criticised his "moral imperialism" in foreign policy at a private dinner, north London weekly newspaper The Islington Tribune said.
The US-led coalition is currently examining its strategy in Iraq in the wake of disastrous mid-term election results for George W. Bush's Republicans and amid mounting violence.
The Iraq invasion has so far claimed the lives of 125 British soldiers, while 2,859 US soldiers have died, according to a recent AFP count based on Pentagon figures.
Security forces were hunting for two Westerners on Saturday who were kidnapped in southern Iraq after an American hostage was found dead and two others rescued.
Britain's finance minister Gordon Brown, widely tipped to take over from Blair as the next prime minister, was in Basra in southern Iraq on Saturday to visit British troops and hold talks with local leaders.
Chancellor of the Exchequer Brown has regularly said that withdrawing troops was not on the short-term agenda.

Copyright AFP (Agence France-Presse), 2006