In the face of a UN Security Council deadlocked on Syria, the United States and its allies could seek other means of legitimising any retaliatory strike they launch against Syria's Government for last week's alleged gas attack on civilians.
The 15-nation council has been split on Syria since 2011. Russia, President Bashar al-Assad's ally, and China have vetoed three resolutions condemning Assad and calling for punitive steps against his Government.
But the US has intervened in conflicts before without Security Council backing, most notably in the Kosovo War in 1999, and could do so again.
Any strike by the United States, Britain, France and others without a clear UN mandate would probably infuriate Russia, which could be expected to denounce it as illegal.
Richard Haas, president of the US Council on Foreign Relations think-tank, rejected the idea - suggested by Russia - that a Western attack on Syria would need UN approval.
''The UN Security Council is not the sole or unique custodian about what is legal and what is legitimate, and, as many have pointed out, it was bypassed at the time of Kosovo,'' he told reporters in a conference call.
''To say only the UN Security Council can make something legitimate seems to me to be a position that cannot be supported because it would allow in this case a country like Russia to be the arbiter of international law and, more broadly, international relations,'' Haas said.
Legitimacy for a strike on Syria, Haas said, could come from a ''coalition of the willing'' supporting retaliation against Assad to demonstrate that the use of weapons of mass destruction will not be tolerated.
That coalition could have formal backing from Nato or other institutions, he said.
US and European officials have cited Nato's bombing campaign intended to pressure Yugoslav President Slobodan Milosevic to withdraw troops and militia from Kosovo. In that case, the US bypassed the Security Council to avoid a Russian veto and got backing from Nato.
The Arab League could formally endorse military action against Syria as it did with Libya in 2011, Richard Gowan, a foreign policy expert at New York University, said. But it might not be easy to secure a consensus in the Arab League and Nato.
US President Barack Obama has tried to distinguish himself from his predecessor, George W. Bush, on foreign policy by presenting himself as more multilateralist. He no doubt would like some kind of international legitimacy if the US attacked Syria.
But the tough tone of comments on Syria by Secretary of State John Kerry and strong suggestions US naval forces are moving into position might mean Obama will go ahead with an attack on Assad's forces no matter what.
White House spokesman Jay Carney on Monday deflected questions about whether Obama would seek authorisation from the UN or the US Congress for military strikes on Syria, saying the president had made no decision on the US response.
Carney repeatedly said Syria's Government had conducted a ''clear violation of an international norm'' by allegedly using chemical weapons against civilians. Russia and Assad blame the rebels for the attack that killed hundreds in Damascus.
Obama has other options for legitimising a retaliatory strike against Syria, apart from securing formal Nato and Arab League endorsements.
Article 51 of the UN Charter speaks of ''the inherent right of individual or collective self-defence if an armed attack occurs against a member of the United Nations.''
In theory, Turkey or Israel could ask the United States and its allies for ''self-defence'' assistance in light of the cross-border violence the two have faced during Syria's two-year civil war.
But Article 51, UN diplomats say, might be difficult to construe as the basis for a response to an attack that did not directly affect any of Syria's neighbours, the US or its allies.
There is also the ''Uniting for Peace'' resolution of 1950, which allows for the UN General Assembly to call an emergency session to take up matters related to international peace and security when the Security Council is deadlocked due to a disagreement between its permanent members.
That resolution enabled the US and its allies to thwart Soviet attempts to use its Security Council veto to cut off support for UN-mandated forces in the 1950-53 Korean War.
UN diplomats say the US would most likely not turn to ''Uniting for Peace.''
But Washington could seek political support from the General Assembly in the form of a non-binding resolution to help legitimise action on Syria.
An assembly resolution could demonstrate that most of the world supports retaliation - provided Washington secured sufficient support in the 193-nation body. All General Assembly votes on Syria have had a majority of nations opposing Assad.
The assembly option had potential, Gowan said. But the assembly option might take time and it is not clear if Obama will be willing to wait if he decides to retaliate. There is also the option of an assembly approval after a strike.
The International Committee of the Red Cross last year described the Syrian conflict as a civil war, which means the Geneva Conventions on warfare apply. Gassing could be a war crime or even crime against humanity, UN diplomats say.
The US and its allies for more than a decade have carried out military actions they say had international mandates, which Moscow has rejected.
Russia regarded Nato operations during the Kosovo war as illegal. It also complained about the 2011 Nato operation to protect civilians in Libya, which led to the death of Libyan dictator Muammar Gaddafi at the hands of rebel forces.
Moscow abstained from a 2011 Security Council vote on Libya, allowing
the first enforcement of the UN doctrine of the ''responsibility to protect'' civilians. Since then it has often cited Libya as a reason for blocking UN action on Syria.
Russia has also cited the 2003 US-led invasion of Iraq as an argument against UN action on Syria. In that conflict, the US assembled a ''coalition of the willing'' to attack Iraq over false allegations about weapons of mass destruction.
