Responding to last week's withdrawal, the Bush administration alleged that not all Syrian intelligence forces had quit Lebanon. It insisted that Syria keep out of Lebanon's elections this month and allow the “disbanding and disarming” of militia forces in Lebanon.

For the moment the US has no plans to send its ambassador, Margaret Scobey, back to Damascus. She was recalled in February after the assassination in Beirut of Rafiq Hariri, the former prime minister. “She will return to Damascus when we feel it's useful for her to return,” a State Department spokesman said.

Officials point out that the US still has a long list of grievances against Syria: its alleged development of chemical weapons and possibly bioweapons; support for militant Palestinian groups; co-operation with Iran in terrorism; its failure to stop Iraqi insurgents using the country as a base; and the shelter it gives to Iraq's former ruling Ba'athists.

Flynt Leverett, a former official in the first Bush administration and author of Inheriting Syria: Bashar's Trial By Fire, said President George W. Bush was moving away from a policy of engagement that had never been properly formulated towards a policy that was “basically regime change”.

More US officials were now inclined in that direction, accepting that forcing Syria out of Lebanon would cause the regime to start to unravel, said Mr Leverett, an analyst at the Brookings Institution. While the US was not gearing up for military action, it believed regime change could be done “on the cheap” through destabilisation.

However, the risk of unintended consequences was very high in both Lebanon and Syria. If the regime in Damascus collapsed and, for the moment, there were signs the withdrawal had made it stronger then the ensuing chaos could lead to a heavily Islamist replacement, Mr Leverett warned.

Yahya Sadowski, of the American University of Beirut, said US policy could be described as “constructive instability”, with the Bush administration believing in general that democracy could emerge out of turmoil.

“The Americans will make what trouble they can for the Syrians, presuming that this will at least reduce Syria's ability to make trouble for anyone else,” Professor Sadowski commented. “If, at some future date, this should trigger political changes in Syria, Donald Rumsfeld [defence secretary] will remind us all that ‘democracy is messy'.”

US diplomats in Damascus insist pressure is aimed at change of behaviour, not regime. But they say Mr Assad has not accepted there is no give and take, and that he has no choice but to deliver.