US Defence Secretary Chuck Hagel said the American military is already prepared to act if President Barack Obama gives the order – though White House aides said no final decision had been taken.
Warplanes and military transporters have been spotted at a British air base on Cyprus, less than 160km from the Syrian coast, The Guardian reported.
Analysts expect to see cruise missiles launched from US and allied submarines, ships and possibly planes, firing into Syria from outside its waters and airspace.
The White House said that any US action would be to defend the principle that chemical weapons should not be used – and would not aim to topple Assad, despite previous calls for him to go.
The Syrian opposition has alleged that the Syrian Army’s Fourth Armoured Division, commanded by the president’s brother Maher al-Assad, was behind last week’s alleged massacre.
The violent and hot-headed military leader has not been seen in more than a year, The Daily Mail reports.
Maher has been at the centre of some of the bloodiest incidents of the conflict so far. It was reported during the uprising in 2011, Maher was spotted shooting at unarmed civilians.
Joshua Landis from the Centre for the Middle Eastern Studies at Oklahoma University said it would be unlikely a command for an attack did not come from the president.
“It’s inconceivable to me,” Landis told Bloomberg. “There has been nothing to indicate that Bashar is just a figurehead.”
Britain and France also moved to back the use of force in Syria, while the White House promised to provide declassified evidence this week to prove that last Wednesday’s chemical attack was the work of regime forces.
Syria has also challenged the United States to prove the attack was perpetrated by regime forces, even as it warned the US it would defend itself against any Western forces.