Britain on Saturday urged Syria's umbrella opposition to reconsider pulling out of international meetings on the crisis, insisting "now is not the time to give up" on talks.
The National Coalition withdrew from the meetings in protest about the international "silence", condemning world powers for failing to stop the slaughter in Syria, as missiles killed at least 29 in the second city of Aleppo.
It said it would pull out of a Friends of Syria meeting in Rome next Thursday and further scheduled talks in Russia and the United States.
Britain said it was "essential" to keep dialogue going between international powers and the coalition, which is trying to oust President Bashar al-Assad.
"The UK is preparing a further offer of support to the National Coalition in Rome, and we are urging them to reconsider their decision," a Foreign Office spokeswoman told AFP.
"It is important to have coalition participation, and we strongly urge members to help win the arguments for achieving more support for the opposition.
"The coalition is making progress; it is not easy, but now is not the time to give up," the spokeswoman said.
"We share a common frustration about the situation in Syria and the horrors being perpetrated by the Assad regime.
"We strongly believe that the international community and the National Coalition need to work closely together to address the situation. That makes it essential that we continue to have a constructive dialogue.
"The meeting in Rome would be a useful opportunity for the coalition and ministers from key supportive countries to continue this dialogue."