LONDON (AP) - Britain's leader said it is legal and just to launch a military strike against Syria even without authorization from the United Nations Security Council, arguing that Syria could repeat its alleged use of poison gas if the international community fails to act.
But Prime Minister David Cameron also seemed to slow Britain's movement toward war, telling legislators in the House of Commons that there is still a sliver of uncertainty over who was behind an alleged chemical attack outside Damascus. He added that Britain would not act if it faced major opposition at the U.N.'s top security body.
"I think it would be unthinkable to proceed if there is overwhelming opposition in the Security Council," he said, without going into detail.
Cameron nevertheless argued strongly for intervention, reminding lawmakers of a series of videos showing the gruesome aftermath of a chemical strike that the rebels and its Western backers blame on Syrian President Bashar Assad, who denies the charges. The independent Doctors Without Borders group says at least 355 people died in the attack.
"The video footage illustrates some of the most sickening human suffering imaginable," Cameron said, adding later: "I think we can be as certain as possible that when we have a regime that has used chemical weapons ... if nothing is done, it will conclude that it can use these weapons again and again, and on a larger scale, and with impunity."
Earlier Thursday his office released intelligence and legal documents meant to bolster the case that retaliation would be justified.
One document, an intelligence assessment, concluded it was "highly likely" that the Syrian government was responsible for the Aug. 21 attack that killed hundreds of civilians, noting there was no credible intelligence to suggest the attack was faked by opposition forces and that no rebel group had the capability for such a large-scale chemical assault. The report did not go far beyond previous public statements, and offered no forensic evidence linking the bombardment to Assad's regime.
Another document, a legal report, was meant to support Cameron's assertion that military action against Syria would be permissible under international law even if it is not specifically authorized by the Security Council. Britain has sought a Security Council resolution but it is opposed by Russia and China, which have veto power.
The UK legal summary says the three necessary requirements for "humanitarian intervention" have been met: There is convincing evidence of extreme humanitarian distress; there is no practical alternative to the use of force if lives are to be saved; and the use of force must be proportionate and aimed at relieving a human crisis.
The documents were made public in advance of a debate in the British Parliament.
Syrian officials Thursday took the unusual step of writing to British legislators denying any role in the attack. In a letter to his counterpart in London, Jihad Allaham, speaker of the Syrian People's Assembly, invited British legislators to come to Syria to investigate the attack.
He implored them to oppose the use of force in Thursday's vote: "We ask you to stop the rush to reckless action," he said, asserting that a military strike would breach international law.
The letter also referred to Britain's experience in Iraq -- a country mentioned time and time again during the debate. Britain, the United States, and their allies went to war there in 2003 on the bogus premise that the country had weapons of mass destruction and was developing links with al-Qaida. In Britain in particular, the invasion and its bloody aftermath still arouses passionate debate.
"The well of public opinion was well and truly poisoned by the Iraq episode and we need to understand the public skepticism," Cameron said.