William Hague today voiced "frustration" at the continuing bloodshed in Syria, but played down the prospect of any foreign military intervention.
But the foreign secretary insisted Britain was doing all it could to try to stop Bashar Assad's brutal attacks on his own people, adding that "time is against" the regime.
The comments came as tributes were being paid to renowned Sunday Times journalist Marie Colvin, who was killed in the bombardment of rebel stronghold Homs yesterday.
Last night the Syrian Ambassador to London, Dr Sami Khiyami, was summoned to the Foreign Office and told the government was "horrified" by the ongoing violence in the city.
Diplomats demanded immediate arrangements to repatriate Ms Colvin's body and for the medical treatment of Sunday Times photographer Paul Conroy, who was injured in the attack.
Speaking on BBC Radio 4's Today programme, Mr Hague said: "It is a deeply frustrating situation... people have been dying in their thousands, that continues.
"The Assad regime has continued to act seemingly with impunity."
He said an international conference in Tunisia tomorrow would seek to agree a "wide set of measures across a large group of nations", and there would be further efforts to bolster UN sanctions next week.
The aim was to tighten the "diplomatic and economic stranglehold" on the Middle East state.
"Do not underestimate the cumulative impact of that over time," Mr Hague added.
"None of (the measures) on their own are the solution, but we are operating under many more constraints than we were in the case of Libya," he said.
Britain wanted Assad "to go", and the economic and other measures were making "life much more difficult" for him.
"Time is against the Assad regime," he added.
However, Mr Hague played down the prospect of any direct intervention, saying Syria's proximity to flashpoints such as Israel and Lebanon made the consequences "much more difficult to foresee".
He also insisted the UK's embassy in Damascus would remain open for the time being, although the situation was kept under "constant review".
"Having an embassy there so far has of course increased our knowledge of what is happening there," he said. "I don't rule out, of course, withdrawing our embassy."