Former vice chief of staff of the US army Jack Keane told the BBC the Americans would "much rather" have UK support for any strikes against Bashar Assad's regime.
General Keane told the BBC that he understood President Obama was planning a more substantial intervention in Syria than had previously been believed, with increased support for the opposition forces, including training from US troops.
UK military action was ruled out by the Prime Minister after the surprise reverse in the Commons last week and Downing Street has firmly played down the chances of a fresh bid to seek MPs' approval for the involvement of British forces.
Gen Keane told the BBC Radio 4 Today programme: "We operate side by side with the UK and we know who our closest ally is. We certainly would much rather do this with the UK side by side, that's how the military feels, I really think the leaders of the country feel.
"I think, if I may use some rich language here, the humiliating defeat the Prime Minister suffered in Parliament, I can only surmise was stunning to the President and I think it impacted on him.
"I think that's one of the motivations that introduced what I call palpable fear and one of the reasons why he is seeking political cover himself."
Obama will seek the backing of the US Congress for action and Gen Keane said the plans could involve "much more substance than we were led to believe".
After speaking to Republican senators John McCain and Lindsey Graham, who were briefed by the President, Gen Keane said: "What he won't do is topple the regime. There's a distinction here.
"What he has told the two senators is that he also intends to assist the opposition forces, so he is going to degrade Assad's military capacity and he is going to assist and upgrade the opposition forces with training assistance."
Gen Keane said any training would probably be done in neighbouring Jordan rather than in Syria itself.
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