Foreign Secretary William Hague has said he was confident the European Union would agree further sanctions to tighten the "diplomatic and economic stranglehold" on the Syrian regime.
Speaking at the inaugural gathering of the 60-nation Friends of Syria group in Tunis, Mr Hague said it was vital not to abandon the Syrian people "in their darkest hour".
Mr Hague joined other representatives at the meeting in recognising the opposition Syrian National Council as "a legitimate representative of the Syrian people".
However he rejected calls for the international community to start arming the rebel Free Syrian Army: "That has not been a subject of this debate. The European Union has an arms embargo on Syria."
The EU has already imposed sanctions on Syrian oil exports, businesses and officials, but French foreign minister Alain Juppe said they would be extended to a freeze on the assets of the Syrian Central Bank.
US Secretary of State Hillary Clinton said President Bashar al Assad's government should allow humanitarian aid into the areas hardest hit by the regime's brutal crackdown, such as the city of Homs.
Qatari Foreign Minister Jassim bin Jabr al-Thani, who has been a driving force to unite Arab opinion against the Syrian regime, joined with Turkey in calling for the creation of humanitarian corridors to get aid to embattled civilians.
Mr Hague said it was essential to intensify the pressure until the bombardment of civilians had ceased, although he acknowledged the progress was "frustratingly slow".
He said Britain would support the Arab League in taking back their peace plan - which would see Assad step down as a precursor to free elections - to the United Nations Security Council. The plan was previously vetoed by Russian and China the last time it was tabled at the Security Council, effectively tying the hands of the international community.
He said that Britain was also documenting the evidence of crimes against humanity in Syria with a view to possible prosecutions in the International Criminal Court.