The United States and its allies are unlikely to build a clear case under international law for a military strike against , and may instead make novel arguments about chemical weapons prohibitions, legal experts have said.
Britain is putting forward a resolution to the security council on Wednesday, condemning the alleged chemical attack in Syria last week and "authorising necessary measures to protect civilians" in the country.
However, Russia, which has a veto on the council, is widely expected to oppose any military action against Syria at the vote in New York. Western powers will therefore need to find another basis – outside of a security council resolution – under which to justify a strike against Syria.
The only other universally agreed basis for military action under international law is self-defence, and it would be hard for the US to argue that the Syrian conflict poses an imminent national security threat.
In Geneva, the UN special envoy to Syria, Lakhdar Brahimi, said on Wednesday that while inspectors had uncovered some evidence of a chemical attack, "international law says that any US-led military action must be taken after" agreement at the 15-nation security council.
"At this point the weight of international opinion would be that military action would not be legal," said Ian Johnstone, a professor of international law at Tufts University. "However, I do think that there could be a case where violation of the law would be excused on the grounds of humanitarian necessity."
Barry Pavel, a former director on the national security council under the Bush and Obama administrations, said that without UN or even Nato support, the US and its allies would seek to justify of a strike on "policy, political, moral and legal grounds".
Pavel, who spent 18 years at the Pentagon is is now a vice-president at the Atlantic Council, said Obama would be expected to make a speech, answering questions about the legal basis of military action, prior to any strikes.
US, British and French political leaders have so far described the expected assault on Syria as a form of punishment or deterrence over its purported use of chemical weapons in the suburbs of Damascus, which resulted in hundreds of deaths.
Senior US officials have repeatedly said that President Assad's forces "flagrantly violated" international law governing the use of chemical weapons, indicating that may form the basis for a justification of any future attack.
Defence secretary Chuck Hagel recently told reporters that any military action would occur "within the framework of legal justification", but stopped short of saying it would be sanctioned under international law.
Professor Matthew C Waxman, from Columbia law school, said that even if the US and allies believed an attack on Syria was justified, they would "undoubtedly face some tough questions about the legality" of an intervention.
He said the US might forgo even attempting a technical justification for military action under international law which, in practical terms, would need the support of China and Russia, and instead build a moral case that use of force was "justifiable and legitimate".
"To set this up, they would put a lot of emphasis on the fact there is a strong legal and moral taboo against the use of chemical weapons," he said.
Legal scholars say that there is no obvious provision in international law that would permit a US-led coalition to mount a limited military offensive against Syria outside authorisation from the UN security council. Indeed, Barack Obama appeared to acknowledge the lack of legal basis for military action.
Pressed on CNN over whether the US would respond to the chemical attacks with force, Obama said: "If the US goes in and attacks another country without a UN mandate and without clear evidence that can be presented, then there are questions in terms of whether international law supports it."
However, international law is fluid and often contested, and the US may attempt to make novel arguments for a strike on Syria, focusing on its use of chemical weapons.
That would represent a departure from previous military actions, which had more conventional bases. The war in Afghanistan was waged on the argument of self-defence, while the air strikes against Libya two years ago were supported by UN security council resolutions. The US invasion of Iraq was not supported by the UN security council.
The nearest precedent for military action on humanitarian or "necessity" grounds would be the 1999 intervention in Kosovo. In that case, the US and Nato forces that bombed the former Yugoslavia accepted there was no conventional basis for military strikes under international law, but insisted there was a moral imperative to prevent genocide.
However, the situation in Syria differs from Kosovo. The US has long resisted intervening in Syria on purely humanitarian grounds, despite a worsening disaster that has so far claimed the lives of more than 100,000 people.
If the US seeks to justify the intervention on humanitarian grounds, saying it has a responsibility to protect Syrian civilians, it will have to explain why it did not choose to act prior to the chemical weapons attack, despite far larger numbers of civilians dying.
There is little evidence the US will seek formal support through Nato. Recent statements from senior administration officials point to the White House focusing on the use of chemical weapons, in what some legal scholars believe could turn into a new interpretation of international law.
Obama said last year that use of chemical weapons by Assad's regime could constitute a "red line". On Tuesday, the White House spokesman Jay Carney said Syria had "flagrantly violated" the norms of the Chemical Weapons convention.
"Because to allow it to happen without a response would be to invite further use of chemical weapons and to have that international standard dissolve," he said.
"And the consequences of that, given the volatility of the region and the concerns that this nation and many others have about proliferation of chemical weapons, would be very serious indeed."
Syria is one of only six countries that have neither signed or acceded to the convention, but it is still likely to be bound to its proscriptions under international customary law. But there is no legal precedent, either in the treaty or elsewhere in international law, that would empower the US to use military action to enforce the chemical weapons law without UN backing.
"The idea that this is some violation of customary law of chemical weapons prohibition is arguably true, but that doesn't automatically mean you can respond with military action," said Johnstone. "Even if Syria had signed the chemical weapons convention, violation of the convention doesn't automatically give other states the right to use force."
Johnstone, from the Fletcher School, said a more sound basis for an intervention, based in part on the responsibility to protect the model that emerged from Kosovo, would be to for a US-led coalition said the extreme humanitarian situation in Syria excused a breach of international law.
However, there may be another dimension to US legal argument. In a key passage, apparently read from his notes, Carney added it was "absolutely in the national security interest of the Unites States and in the international community" to ensure the violation was not ignored.
Waxman, who previously held senior positions at the Department of Defense and national security council, said the administration suggested it may argue that military action is "necessary in order to enforce" the prohibition of chemical weapons.
"A criticism of that argument would be that, while it is true that there is a very strong legal and moral taboo against the use of chemical weapons, it is not clearly established that the appropriate remedy for breaching that taboo is military force."
Michael Glennon, also from Tufts, said the implication in Carney's remarks that a chemical weapons attack that went unpunished could threaten US national security, came close to "an argument for preventive war", which was controversially aired by some in George W Bush's administration.
"Many people in the Obama administration criticised the Bush administration for arguing in favour of the legitimacy of preventive war," he said.
However, Glennon said arguments of international law could prove academic; the framework for use of military force, as set out in the UN charter rules, is often ignored. "They have come to mean little or nothing," he said. "They've been violated somewhere between 200 and 680 times since 1945, and they've come to look like a piece of Swiss cheese."
"I don't think the administration is wrong to say there is nothing in international law that prohibits it from firing cruise missiles through the windows of the Syrian defence ministry. The question, in a consent based system, is always what are the rules that states consent to?"
Glennon added: "Behind closed doors the decision-makers know the rules are malleable and can be shaped to what they need to support the policy decisions. It is telling that the policy decision seems already to have been made – they're only now looking for the legal rationale."
• The article was amended on 28 August to correctly reflect that the US invasion of Iraq was not supported by a UN security council resolution.