US Secretary of State Hillary Clinton said Monday that the United States is "hoping for the best" but is discussing next steps with other powers if the ceasefire in Syria collapses.

"Much of Syria is quieter... We know the ceasefire is not complete but it appears as though the violence is down significantly," despite renewed violence in the city of Homs, Clinton told reporters during a visit to Brazil.

But Clinton said Syrian President Bashar al-Assad must not only comply with a ceasefire, he must also comply with UN and Arab League envoy Kofi Annan's six-point peace plan and a UN resolution sending monitors to oversee it.

"What the Assad regime needs to do is to make clear that they're going to silence their guns, withdraw their troops and work toward fulfilling the six-point plan," the chief US diplomat said.

Complying with the Annan plan also means allowing peaceful demonstrations, releasing political prisoners and allowing a peaceful political transition to begin, Clinton said.

"So this week will be critical in evaluating the implementation of the United Nations Security Council resolution" that provides for the dispatch of the monitors to observe implementation of the peace plan, she said.

"And we're hoping for the best. We want to see a peaceful period for the people of Syria," Clinton said.

"We want to see a political process begin, but if violence is renewed, the regime reverts to shelling its own people and causing a great deal of death and injury, then we're going to have to get back to planning what our next steps" are, she said.

"We're planning for a good outcome, positive results, and we're talking with others on the security council and beyond what would be next steps if that does not prove successful," she said.

"The burden is on the Assad regime to demonstrate their commitment to all aspects of Kofi Annan's six point plan," she said.

"We are not interested in new promises, we're not interested in new conditions or new excuses. We want action," she said.