WASHINGTON (AP) - President Barack Obama says the United States will keep up pressure on Syria's leader to stop the "slaughter" of civilians.

Obama says it is "imperative" that the world unite in its condemnation of the Syrian military onslaught on civilians. Civilians are under siege in the restive city of Homs, among other places, and thousands have died. Obama says the world cannot be a bystander to the violence, but he did not give specifics about what the U.S. or other countries could really do to help.

Obama says "it is time to stop the killing of Syrian citizens by their own government."

Obama spoke in Washington shortly after Secretary of State Hillary Rodham Clinton used strong language to denounce Russia and China for protecting Syria..