Secretary of State Hillary Clinton says the US is preparing for the potential failure of former UN chief Kofi Annan's plan to end the violence in Syria and will take additional steps against the Syrian government if it does.

Clinton on Tuesday condemned reports that UN ceasefire monitors have been unable to do their job properly and that violence is continuing. She said the US and its allies are committed to Annan's plan and want it to succeed. But she said there is no faith that President Assad's government will comply with it.

Anticipating failure, Clinton said work was under way to come up with additional US sanctions to further punish and isolate the Assad regime.