WASHINGTON (AFP) - The United States (US) hinted on Thursday it could act alone to punish Syria for a chemical weapons attack, following hesitation by its closest ally Britain and a deadlock at the United Nations (UN).
The Obama administration also denied that public skepticism dating to an Iraq war intelligence debacle was complicating its effort to justify possible military action against Syria.
National security heavy-hitters including the secretaries of state and defence, Mr John Kerry and Mr Chuck Hagel, planned to brief top lawmakers on possible responses to the attack in a Damascus suburb last week that killed hundreds of people.
The intelligence community was working meanwhile on a declassified public report on the attack, which officials said will show there is no doubt that President Bashar al-Assad's regime shattered taboos by using chemical weapons on civilians.
US President Barack Obama, who came to power criticising his predecessor George W Bush's go-it-alone approach on foreign policy, was confronted on Thursday with a choice over whether to wait for allies or launch unilateral US action.
The White House said that while Mr Obama prized the UN and closely consulted allies, his first duty was to US national security, which he sees threatened by the Syrian attack.
"The President's chief accountability is to the American people that he was elected to protect," said White House spokesman Josh Earnest.
The comments came as Britain struggles for a political consensus over Prime Minister David Cameron's plans to join expected US-led military action.
The political fracas in London sparked speculation that a timetable for action, which many observers had believed could see air strikes in Syria within days, could slip.
But at the State Department, spokesman Marie Harf said: "We make our own decisions in our own timeline."
The Obama administration also hinted that unlike Britain, it did not see the need to wait for a report by UN inspectors in Syria on the chemical attack on a Damascus suburb on Aug 21.
It reasons that since the panel's mandate is merely to establish an attack happened, and not to apportion blame, its findings are moot.
On Wednesday, the Obama administration said it did not see any future in a British bid to secure a mandate from the UN Security Council for attacking Syria, due to Russian opposition.
Mr Obama sees perils to US national security in the belief that Syria shattered international norms by using chemical weapons, and that US interests and regional allies could be threatened next.
The release of the document detailing the Syrian attack was expected to come Thursday, though officials said it had not yet been finalised.
Obama aides stress they envisage only "limited" punitive action in Syria and dismiss comparisons with the US invasion of Iraq, which the President built his political career on opposing.
The discrediting of what was once deemed "slam dunk" intelligence on Iraq's weapons of mass destruction is imposing a high burden of proof on the current administration.
Center for Strategic and International Studies analyst Anthony Cordesman termed the coming report the "most important single document in a decade". But officials denied the Syria drama was in any way comparable to Iraq.
"Nobody needs an intelligence community assessment to know that chemical weapons were used here," said Ms Harf.
"In Iraq, we were waiting for an intelligence community assessment to determine whether they even existed. Those are two categorically different levels of assessment being done here."
Some reports this week suggested that the US assessment would include communications intercepts and other data from inside Syria.
But sources are downplaying expectations, saying the public analysis will mirror the case already made by Washington, which they consider strong.
Signs that the US report will not be able to directly link Mr Assad to the attack with anything other than circumstantial evidence were bolstered by a British intelligence report released on Thursday.
The study found no credible intelligence to suggest opposition forces fired the chemical arms and that there was no "plausible alternative" to the idea that the regime was to blame.
But, possibly due to the need to safeguard sensitive intelligence sources, there was no detailed evidence directly implicating Mr Assad.
While Mr Cameron recalled parliament for an uncomfortable debate on Syria, there were no plans for the US Congress, which is in recess, to do likewise.
The White House will likely argue that since its proposed action in Syria will be "limited," it does not require Congress to wield its constitutionally granted power to authorise a declaration of war.
But the more time that passes before US military action, the more restive the domestic political scene becomes.
Mr Earnest said Thursday's consultation exercise with lawmakers would likely not be the last one.