WASHINGTON (AFP) - Signs are growing that the horror of chemical warfare could for the first time draw the United States into a direct military intervention in Syria's vicious civil war.
The US military and diplomatic machine is slowly stirring, after pictures of children apparently choked on poisonous gas shocked the world last week.
President Barack Obama is facing another test of a legacy doctrine rooted in avoiding Middle Eastern quagmires.
Syria touches a perennial question of whether humanitarian impulses or narrow national interest should define US foreign policy. The crisis is intensifying as Mr Obama is wrestling with how to respond to a coup in Egypt.
The president held a rare Saturday meeting with top aides including Vice President Joe Biden, his secretaries of defense and state, intelligence chiefs and senior brass to discuss the US response.
Then he called British Prime Minister David Cameron - hinting at an effort to frame an international coalition for action.
It is unclear whether Mr Obama is leaning towards a military strike if the use of chemical arms by Assad's troops, which would infringe a "red line" he established last year, is proven. It is possible that US diplomatic and military activity in recent days could be simply designed to build pressure on Syrian President Bashar al-Assad, who has resisted all US demands to quit power.
But the administration appears to be taking steps that would be expected to precede a decision to target Syrian units eventually implicated in the attack outside Damascus.
The Pentagon is positioning forces, including ships equipped with cruise missiles, closer to Syria while Obama aides examine the Kosovo conflict for legal precedents for action without a UN mandate, which Russia would surely block.
Secretary of State John Kerry has also been burning up phone lines, talking to US allies in Europe and the Middle East.
Julian Zelizer, a Princeton University foreign policy historian, believes Mr Obama is yet to sign off on military force.
But he said the administration clearly "feels pressure to do something, or to look like it is preparing to respond, for political reasons, (and to show Syria) that there are boundaries to what is permissible."
Obama aides caution that no decisions have been finalised, and want definitive proof that Syrian forces strafed a rebel-held Damascus suburb with chemical arms and killed 1,300 people.
But the White House offered credence to the reports in a statement on Saturday's meeting, noting "contemporaneous witness accounts and (a) record of the symptoms of those killed."
Simply by being seen to marshall the US response, Mr Obama is raising the stakes.
"There is a sense in Washington that this is new territory and they will take some sort of action," said Barry Pavel, a former senior defense and national security official in the Clinton, Bush and Obama administrations.
But if robust action does not materialise after Washington pumped its muscles, the administration risks tarnishing its credibility, said Mr Pavel, now with the Atlantic Council.
Mr Obama's personal reputation is also in play, after he said last year the widespread use in Syria of chemical weapons would cross a US red line.
US foes in Iran, China and North Korea are watching to see whether defying the US president entails a price.
Mr Obama responded to previous small scale chemical weapons use in Syria by promising direct aid to rebel groups.
But he called the chemical attack a "big event", raising the stakes for himself. A US intelligence finding that the Assad regime was indeed to blame would put Mr Obama in a tough political spot.
Officials have only spoken in broad terms about possible military options.
Any strike would have to be strong enough to deter reprisals by Mr Assad's military, but sufficiently limited to avoid pulling Washington further into the war.
Hussein Ibish, a Middle East analyst with the American Task Force on Palestine, believes military action is possible.
"It seems to me the minimum action we would expect is a limited cruise missile attack on some chemical weapons-related sites.
"It could include disruption of facilities that are useful to the regime."
Whatever happens, Mr Obama wants to be remembered as a president who ended US wars, not one who opened new fronts.
Officials say there is no talk of a no-fly zone in Syria. And there are also signs the administration is concerned about framing a legal rationale for any action.
One blueprint could be the coalition, including European and Gulf states, that went to war in Libya to protect civilians.
That scenario would lack the United Nations Security Council resolution on Libya - which Russia did not block, only later to conclude it had been duped into regime change.
Mr Obama must also consider what would happen if military action went wrong, including the danger of civilian casualties. Domestic politics also cannot be ignored.
Mr Obama would likely argue that limited action against Syria would fall short of the scale of operation requiring a formal authorisation of war for Congress.
