Britain's government has published internal legal advice which it said showed it is legally entitled to take military action against Syria, even if the United Nations Security Council blocks such action.
It also published intelligence material on Thursday on last week's alleged chemical weapons attack in Syria, saying there was no doubt that such an attack had taken place.
The document is the latest sign that a coalition of Western countries, including the United States, France and the UK, are moving towards military action against Syria after the alleged attack.
It was "highly likely" that the Syrian government was behind the attack, the document said.
"If action in the Security Council is blocked, the UK would still be permitted under international law to take exceptional measures in order to alleviate the scale of the overwhelming humanitarian catastrophe in Syria," a copy of the British government's legal position read.
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In such circumstances, it added that "military intervention to strike specific targets with the aim of deterring and disrupting further such attacks would be necessary and proportionate and therefore legally justifiable".
In a debate on Thursday, however, Prime Minister David Cameron told the British parliament it was "unthinkable" that Britain would launch military action against Syria if there was strong opposition at the Security Council.
"It would be unthinkable to proceed if there was overwhelming opposition in the Security Council," he said.
Al Jazeera's Simon McGregor-Wood, reporting from London, said Cameron made a "passionate performance" in parliament.
"He made several and repetaed references to mistakes which were made during the Iraq war, to give a message that 'this is not Iraq,'" he said.
"Cameron also emphasised the danger of doing nothing."
The five permanent members of the Security Council were scheduled to meet again on Thursday to discuss the crisis. The UK and US have both already dispatched planes and warships to the eastern Mediterranean.
Earlier on Thursday, Syrian President Bashar al-Assad said that his country would defend itself against any foreign military intervention.
"Syria will defend itself in the face of any aggression, and threats will only increase its commitment to its principles and its independence," the embattled Syrian leader told a visiting delegation of Yemeni politicians, according to state media.
The White House said on Thursday that it had strong circumstantial evidence linking Assad's government to an alleged chemical weapons attack in the East Ghouta suburbs of Damascus last week, which aid agencies say killed at least 355 people, and injured as many as 3,000 others.
The Syrian army "has been engaged in a military campaign targeting the areas where these attacks occurred," said Josh Earnest, a White House spokesman. But he did not cite any direct evidence showing that Assad or his top lieutenants ordered the attack.
The Syrian government has strongly denied accusations that it was involved.
The most likely option for an attack, US officials say, would be to launch cruise missiles from US ships in the Mediterranean in a campaign that would last several days.
New hurdles have, however, emerged that appear to have slowed the formation of an international coalition that could use military force to hit Syria.
On Wednesday, the five permanent members of the United Nations Security Council failed to reach an agreement on a draft resolution from the British seeking authorisation for the use of force.
Russia objected to international intervention, after Foreign Minister Sergei Lavrov earlier rejected the case for ascribing culpability to the Syrian government at this time, adding that foreign military intervention would lead to "destabilisation of [...] the country and the region".
Chinese state media on Thursday said that any military intervention "would have dire consequences for regional security and violate the norms governing international relations".
Source: Al Jazeera and agencies