Tony Blair has accused Iraq of "executing" the two British soldiers whose bodies were shown on Arabic television as he held talks with US President George Bush
Blair says "an enormous amount" has been achieved
Mr Blair made the claim, which is denied by Iraq, at a news conference with Mr Bush at Camp David near Washington.
"If anyone needed any further evidence of the depravity of Saddam's regime,
this atrocity provides it," he said.
"It is yet one more flagrant breach of all the proper conventions of war.
"More than that, to the families of the soldiers involved, it is an act of cruelty beyond comprehension.
"Indeed, it is beyond the comprehension of anyone
with an ounce of humanity in their souls."
Al-Jazeera television has screened pictures of two dead UK troops who went missing in an Iraqi ambush on Sunday.
Asked why he had referred to their deaths as executions, Mr Blair said it was "because of the circumstances we know".
Speaking later, the prime minister's official spokesman acknowledged there was not absolute proof they were executed but claimed that "every piece of information points towards the men having been executed in a brutal fashion".
He said the bodies had been found some distance from their vehicles and their protective equipment and helmets were missing.
Minister Mohammad Saeed al-Sahaf told Abu Dhabi television that Mr Blair had "lied to
the public" about the soldiers, adding: "We haven't executed
The two British soldiers who went missing on Sunday, and are believed dead, have now been named as Sapper Luke Allsopp, 24, and Staff Sergeant Simon Cullingworth, 36, both from a bomb disposal unit of the Royal Engineers.
The prime minister said the priority for the US-led coalition had to be military victory, but he stressed the role of the United Nations in endorsing Iraq's post-war future.
He said "an enormous amount" had been achieved in the conflict so far.
Asked about how long the conflict would take, President Bush said: "As long as it takes."
Mr Blair is later due to meet United Nations Secretary General Kofi Annan in New York.
Sir Jeremy Greenstock, British ambassador at the UN, said UK wanted the UN to play a "central role in the future of Iraq" and the prime minister would discuss those hopes with Mr Annan.
At their joint news conference, President Bush said: "Slowly but surely the grip of terror around the throats of the Iraqi people is being loosened."
Commentators say the US and UK are split over the future role of the UN, with the UK saying Iraq's post-war administration should be endorsed by a new UN resolution.
Mr Bush gave no commitment to backing an interim UN-led administration in Iraq in the future.
He said: "Iraq's greatest long-term need is a representative government that protects
the rights of all Iraqis.
"The form of this government will be chosen by the Iraqi people, not imposed
On Wednesday, US Secretary of State Colin Powell, said the UN should have a role in Iraq's future - but ruled out handing over control to the body.
After a coalition victory, there would first be US military rule to stabilise the country, before passing to an American civil administration, said Mr Powell.
There would then be an interim Iraqi administration, which would work alongside a UN co-ordinator, he said.
Other figures in the White House are thought to be pushing for the UN to be limited to a humanitarian role.
Mr Blair said it was too early to discuss the details of the UN's role in the future or indulge in "megaphone diplomacy" on the issue.
He said there were "no differences" between the US and the UK.
Both leaders called for a UN resolution to restart the oil-for-food programme - an issue which President Bush said should not be "politicised" - in Iraq.
The prime minister warned against speculating about how long the conflict in Iraq could take.
But he said: "Just under a week into this conflict, let me restate our complete and total
Mr Blair told the Iraqi people: "We will liberate you. The day of your freedom