US Secretary of State John Kerry on Monday urged the Syrian opposition to reconsider its threat to boycott talks with foreign powers due in Rome this week.
"I would urge the Syrian opposition to join us as a matter of practicality and of informing us," Kerry said in a news conference in London after talks with British Foreign Secretary William Hague.
He promised the rebels that the Friends of Syria group was not meeting on Thursday merely to talk but "we are coming to Rome to make a decision on next steps".
He stressed though that the United States continued to pursue a political resolution to the conflict.
Kerry said the killings in the Syrian city of Aleppo last week were "further evidence" that President Bashar al-Assad must stand down.
"The Syrian people deserve better than the horrific violence that now invades and threatens their everyday lives," Kerry said.
The Syrian National Council has said it will pull out of the 11-nation Friends of Syria talks in protest at the international community's "shameful" inability to halt the bloodshed.