WASHINGTON -- The United States is preparing legal justification for the use of force against Syria with allies Britain and France that would circumvent the United Nations Security Council, where Russia and China have vowed to block any resolution authorizing military intervention in the conflict.

The US will detail its case soon, with military action possible in the coming days, sources told The Jerusalem Post on Monday.

On Sunday, the US dismissed Syria's decision to grant UN inspectors access to the the Damascus suburb of Ghouta, where Syrian rebels claim the army of nominal president Bashar Assad used sarin gas last week to kill hundreds of civilians as they slept.

UN inspectors sat in their hotel rooms for five days after the attack, just five miles from the scene. The investigation that started Monday would be "too late to be credible," the US said, as evidence of sarin lasts for a maximum of ten days.

Russia has repeatedly warned the West against action in recent days, citing international law as prohibitive to any trilateral campaign. Last week, US President Barack Obama stated that action in Syria "without a UN mandate" might indeed violate international law.

But the Obama administration has proven creative in its legal justifications in the past. Faced with a domestic law requiring the suspension of aid to any country that has faced a military coup d'etat, the president made the determination not to make a determination on whether a coup had occurred in Egypt in July, after the military forcibly removed its president, Mohammed Morsi, from power.

Administration officials have been studying Western intervention in Kosovo as a model for action in Syria, The New York Times reports.

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Speaking in Indonesia, US Defense Secretary Chuck Hagel refused to detail military contingencies prepared by the Pentagon for the president.

"We are analyzing the intelligence," Hagel said. "And we will get the facts. And if there is any action taken, it will be in concert with the international community and within the framework of legal justification." 10 Downing Street told Britain's Parliament that Prime Minister David Cameron reserved the right to take action without parliamentary approval on Monday, as lawmakers there considered returning to London early from summer recess. Britain's foreign minister William Hague told local news outlets that the UK did not see Security Council approval as a requirement for action.

"Otherwise it might be impossible to respond to such outrages, such crimes, and I don't think that's an acceptable situation," Hague said when asked about unanimous Security Council approval by BBC Radio.

Britain has readied Royal Navy forces in the Mediterranean to strike with American destroyers should their leaders choose to move forward. A strike would likely involve standoff assets that would avoid entry into foreign air space.

Senator John McCain, anticipating a limited, symbolic strike on Assad's command and control centers, said Sunday that the president should consider "very serious action, not just launching some cruise missiles." France's Francis Hollande on Sunday said that his country's intelligence agency had gathered corroborating evidence that the Assad regime had used chemical weapons against civilians on August 21. US officials have said there is "little doubt" that an attack occurred, and was perpetrated by Assad forces.

The State Department has repeatedly asserted that the US does not believe rebel forces in Syria are capable of either making or dispersing chemical agents on a mass scale, much less sarin, which is difficult to handle.

Sarin gas can evaporate quickly after release, especially under hot conditions such as August in Syria. Western officials were also concerned when Assad forces continued shelling the scene with conventional artillery after Wednesday's alleged chemical attack, perhaps in an effort to destroy evidence.

Syrian rebel leaders told Reuters on Monday that peace talks were no longer on the table after the Wednesday's attack.

Syrian National Coalition secretary-general Badr Jamous said they "refused to speak about [peace talks in] Geneva after what's happened," called Assad "Bashar the Chemist." Meanwhile, the UN team pushed forward with its investigation on Monday, gathering evidence while under sniper fire in the Damascus province.

A sniper hit a UN inspector's SUV "deliberately" and "multiple times," the UN said in a statement, calling the incident "outrageous." The team promptly replaced its vehicle and continued with its inspection work.

A UN spokesman told the Post on Sunday that the team "was not making any predictions" how long its investigation would take.

"We cannot allow impunity in what appears to be a grave crime against humanity," UN secretary-general Ban Ki-moon said of the crisis on Sunday.

International law bans the use of chemical weapons on any battlefield under any circumstances. And the "responsibility to protect," or R2P— a norm agreed upon by global powers at the United Nations 2005 World Summit— compels the international community to respond if a country fails to protect its citizens from genocide, war crimes, ethnic cleansing or crimes against humanity.

R2P is not law. But its principles are rooted in law, and have been agreed upon by the international community, including by Russia, which has cited R2P in the past when waging military campaigns in the caucasus.

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