US secretary of state Hillary Rodham Clinton is calling for "friends of democratic Syria" to unite and rally against President Bashar Assad's regime.

Speaking on Sunday in the Bulgarian capital Sofia, the day after Russia and China blocked UN Security Council action on Syria, Clinton said the international community had a duty to halt ongoing bloodshed and promote a political transition that would see Assad step down.

Previewing the possible formation of a group to coordinate assistance to the Syrian opposition, she said the "friends of Syria" could work together to promote those goals.

Such a group could be similar to the Contact Group on Libya, which oversaw international help for opponents of the late deposed Libyan leader Muammar Gaddafi.