US president Barack Obama has said he will keep up the pressure on Syria's leader Bashar Assad to stop the "slaughter" of civilians.

Mr Obama said it is "imperative" that the world unites in its condemnation of the Syrian military onslaught on its people.

Thousands of people have died as civilians were fired upon in the restive city of Homs, among other places.

Mr Obama added the world cannot be a bystander to the violence, but he did not give specifics about what the US or other countries could do to help. The US president said "it is time to stop the killing of Syrian citizens by their own government".

He spoke in Washington shortly after US secretary of state Hillary Clinton used strong language to denounce Russia and China for protecting Syria.

Mrs Clinton blasted Russia and China's opposition to UN Security Council action on Syria, calling it "despicable". She said she is willing to go back to the UN as often as needed, but added: "We need to change the attitude of the Russian and Chinese governments."

She spoke at an international conference in Tunisia aimed at persuading Assad and his allies into accepting demands for a democratic transition.

Mrs Clinton added that members of Mr Assad's own security forces would overthrow him if forced to continue targeting civilians, while many rebel soldiers have already defected to the opposition side.

"We also know from many sources that there are people around Assad who are beginning to hedge their bets - they didn't sign up to slaughter people," she told the conference. "Their refusal to continue this slaughter will make them heroes in the eyes of not only Syrians, but people of conscience everywhere."

Mrs Clinton noted that in several other cases over the past year, military forces have stepped in to remove unpopular leaders, as happened in Tunisia and Egypt. "We saw this happen in other settings last year, I think it is going to happen in Syria," she said.