William Hague has said he is confident the European Union will agree further sanctions to tighten the "diplomatic and economic stranglehold" on the Syrian regime.
It comes as efforts continue to rescue injured British photographer Paul Conroy and recover the dead body of his Sunday Times colleague Marie Colvin after they were caught in the midst of a shell attack in the besieged city of Homs.
Speaking at the inaugural gathering of the 60-nation Friends of Syria group in Tunisia, the Foreign Secretary said it was vital not to abandon the Syrian people "in their darkest hour".
He joined other representatives in recognising the opposition Syrian National Council as "a legitimate representative of the Syrian people". But rejected calls for the international community to start arming the rebel Free Syrian Army.
Friday's international gathering came as The International Committee of the Red Cross began to evacuate those caught up in the violence from the besieged Homs neighbourhood of Baba Amr.
It is believed that Mr Conroy, 47, was not among the 27 people rescued from the area by local workers. He was injured and his Sunday Times colleague Marie Colvin, 56, was killed in a rocket attack by Syrian government forces on the makeshift media centre where they were working on Wednesday.
Speaking in Tunis, Mr Hague argued that it was essential to intensify the pressure on the Syrian regime until the bombardment of civilians had ceased. Acknowledging the progress was "frustratingly slow" he added: "By miring itself in the blood of innocent people the Syrian regime has forfeited the right to lead. We will use all diplomatic and economic means to stop the activities of what has now become a criminal regime."
The EU has imposed sanctions on Syrian oil exports, businesses and officials, and are expected to extend them to a freeze on the assets of the Syrian Central Bank.
Mr Hague said Britain was documenting the evidence of crimes against humanity in the country with a view to possible prosecutions in the International Criminal Court. "Those responsible for the murder of entire families, the shelling of homes, the execution of detainees, the cleansing of political opponents and the torture and rape of women and children must be held to account," he said. "They should be no hiding place for those committing crimes."
He added that 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. However, Mr Hague said Moscow and Beijing should not wield their vetoes again.