The question now becomes what the United States, probably with France and Britain, will hit in war-torn Syria, and what are the risks of escalation and miscalculation with Assad allies Russia and Iran.

- What will it target? -

The United States and its allies France and Britain want to send a clear and definitive message to Assad that the future use of chemical weapons will not be tolerated.

One way to ensure Assad cannot use such weapons is to conduct strikes on his chemical infrastructure, including by targeting factories and weapons holding areas.

However, such an action carries risks, including the possibility of unintentionally unleashing a cloud of toxic gas into the environment.

The strike could also target what is left of Assad's air force and air defenses, though the delay between Saturday's attack and an eventual retaliatory strike means he likely will have moved his warplanes to safety.

Jennifer Cafarella, a Syria analyst for the Washington-based Institute for the Study of War, told AFP that one potential target would be Dumayr military airport northeast of Damascus, from where regime aircraft used in Saturday's attack may have launched.

- How big will the strike be? -

Trump last year ordered a strike against Assad following a deadly sarin gas attack on the rebel-held town of Khan Sheikhun.

The US Navy launched 59 Tomahawk cruise missiles from destroyers in the eastern Mediterranean, targeting Shayrat air base, its aircraft, ammunition supply bunkers, air defense systems and radars.

But the strike was limited and designed to not draw America into the chaos of Syria's bloody civil war.

Since then, the regime is alleged to have repeatedly used chlorine and other chemicals, forcing Pentagon planners to consider broader action.

Cafarella said the United States must decide how far it wants to go in terms of hitting Iranian proxy forces near US positions in eastern Syria, or even targeting Russian-supported facilities.

"The president has publicly stated that he intends to hold not just Assad but his backers accountable as well," she noted.

- How will it be launched? -

A strike against Syria will likely come in the form of missiles, as was the case last year.

The United States would not want to risk putting manned aircraft over Syrian air defenses -- a shoot-down would send the conflict spiralling in unforeseeable new directions.

The USS Donald Cook, an Arleigh Burke-class guided-missile destroyer, is within easy striking range of Syria, as is a French frigate with its own cruise missiles.

These two ships, possibly aided by a US submarine, are likely to play a role in a strike.

- What are the risks? -

The reaction from Assad backer Moscow is unpredictable and Russia has threatened retaliatory action against the United States if missiles are fired at Syria.

The Russian army on Wednesday accused the White Helmets civil defense organization of staging a chemical weapons attack in Douma, where observers say more than 40 people died in a gas attack.

NBC News reported Tuesday that Russia has learned how to use GPS jammers to limit the capabilities of US drones operating over Syria.

"The US has to be very careful not to accidentally strike Russian targets or kill Russian advisors," Ben Connable, a senior political scientist at the RAND Corporation, told AFP.

"That significantly limits the number of options available to the United States, because the Russians are embedded in many cases with the Syrians."

Connable warned that if the US accidentally or purposefully kills uniformed Russian soldiers, there would potentially be a dangerous escalation between the two nuclear powers.