The Syrian military has the capacity to hit American and British bases in the region using its arsenal of Scud B missiles, but analysts believe Bashar al-Assad's regime is more likely to respond indirectly to any Western strike.
This could entail staging attacks on neighbouring American allies such as Turkey and Jordan, or even action by groups supported by Syria's key ally and regional power, Iran.
"They [the Syrian regime] don't want a new front, a new war," said Joshua Landis, a Syria academic with good contacts in Assad's ruling Alawite sect. "A third-party assassination, or a car bomb or two, that is much more likely."
Targets for a Syrian reprisal could include:
Military bases of nations which join attack
The most obvious would be the US airbase at Incirlik, in southeastern Turkey, or RAF Akrotiri, part of the British military base in Cyprus. Both are within the 300km range of the missiles which could be fitted with chemical weapons.
Targeting military bases would risk a Western decision to "finish off the job" and launch a full-scale attack.
Assad could follow the example of Saddam Hussein, who launched Scud missiles at Tel Aviv during the Gulf War. The Israeli Cabinet authorised a partial call-up of reservists in the north yesterday. Its military said it was deploying its missile shield systems, although the Prime Minister, Benjamin Netanyahu, said there was "no reason for a change to normal routines".
Syria's rebels or civilians
The regime has responded to rebel advances in the past by bombarding the civilian population behind rebel lines. That would endorse the arguments of critics of Western policy that intervening only makes matters worse for ordinary people. However, attacks on rebel areas would hardly be distinguishable from the status quo.
Facilities used by Westerners in Jordan
Five-star hotels in the capital Amman have been targeted in the past by al-Qaeda. Jordan is a close ally of the United States and dependent on aid from Saudi Arabia, which is supplying weapons to the rebels.
The Turkish border
Turkey's Prime Minister, Recep Tayyip Erdogan, is an anti-Assad cheerleader, and allows rebels and weapons to pass through the border. But a recent car bombing in the border town of Reyhanli that was blamed on the Assad regime triggered protests by locals unhappy with Erdogan's Syria policies. More attacks would undermine his authority and send a warning to the US that Syria could threaten the security of a key Nato member.
Rocket attacks on Israel by Iranian proxies
One of those is the Lebanese Shia militia Hizbollah, though it might be wary of being further drawn into a conflict in which its members are already fighting. Another is the Gaza-based Hamas, but it has fallen out with both Iran and the Assad regime, having come down on the side of the rebels. However, Iran also sponsors the Gaza-based Islamic Jihad.
AP quoted intelligence officials as saying the intelligence linking Assad or his inner circle to the Ghouta attack is no "slam dunk," with questions remaining about who actually controls some of Syria's chemical weapons stores and doubts about whether Assad himself ordered the strike.
The complicated intelligence picture raises questions about the White House's full-steam-ahead approach to the attack, with worries that the attack could be tied to al-Qaeda-backed rebels later.
Intelligence officials say they could not pinpoint the exact locations of Assad's supplies of chemical weapons, and Assad could have moved them in recent days. US satellites have captured images of Syrian troops moving trucks into weapons storage areas and removing materials, but US analysts have not been able to track what was moved or, in some cases, where it was relocated. They are also not certain that when they saw what looked like Assad's forces moving chemical supplies, those forces were able to remove everything before rebels took over an area where weapons had been stored.
An intercept of Syrian military officials discussing the strike was among low-level staff, with no direct evidence tying the attack back to an Assad insider or even a senior Syrian commander, the officials said. All sides accept that a chemical attack took place in Ghouta last week, but Syria and Russia say rebel forces were responsible.
- additional reporting, AP