The reported flight to the UK of Syria's foreign ministry spokesman is being investigated by UK officials.
A Foreign Office spokesman said it was aware of claims. attributed to Lebanese security officials, that Jihad Makdissi had taken a flight to Britain.
It remained unconfirmed whether Mr Makdissi, previously the spokesman for the Syrian embassy in London, had defected from Bashar al-Assad's regime.
He was a prominent public defender of the Damascus government's response to the 20-month uprising but was reported to have been sacked.
The reports emerged as the United Nations announced it was withdrawing all "non-essential international staff" from Syria amid deteriorating security conditions.
And the US issued fresh warnings to the regime against using chemical weapons after a senior US defence official warned that movements of components had recently been detected.
US secretary of state Hillary Clinton echoed President Barack Obama's caution that such action would cross a "red line" for the United States and prompt action, though she did not specify what.