U.S. Secretary of State Hillary Rodham Clinton is calling for "friends of democratic Syria" to unite and rally against President Bashar Assad's regime.

Speaking Sunday in the Bulgarian capital of Sofia, the day after Russia and China blocked U.N. 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, but not identical, to the Contact Group on Libya, which oversaw international help for opponents of the late deposed Libyan leader Moammar Gadhafi.