President Obama on Saturday condemned the latest attack by the Syrian regime on its own people and once again called for President Bashar al-Assad to step down immediately.

But a U.S.-led effort for a United Nations resolution calling for Democratic government in Syria was blocked in the Security Council by China and Russia. 

Obama’s statement comes after reports that Syrian troops have killed hundreds in Homs, a stronghold of protesters against his regime. The attack came on the 30th anniversary of a massacre in the city of Hama by Assad’s father.

{mosads}“I strongly condemn the Syrian government’s unspeakable assault against the people of Homs and I offer my deepest sympathy to those who have lost loved ones,” Obama said. “Assad must halt his campaign of killing and crimes against his own people now.  He must step aside and allow a democratic transition to proceed immediately.”

Obama said that Assad “has no right to lead Syria.”

His statement came as the United States was trying to negotiate a strongly worded condemnation from the United Nations Security Council. 

In a 13-2 vote by the U.N. Security Council later on Saturday, however, Russia and China blocked a U.S.-sponsored resolution that would have backed a transition to democracy but did not call for regime change explicitly. That transition process was devised by the Arab League. 

France’s ambassador said that those blocking the resolution were complicit in the violence against protesters, but opponents said the resolution could lead to armed ouster of Assad. 

“The United States is disgusted,” U.S. Ambassador Susan Rice said of the outcome. She said the U.N. was being “held hostage” by China and Russia.

She told the council that the two members would stop at nothing to “sell out the Syrian people and support a craven tyrant.”

“The Council now has an opportunity to stand against the Assad regime’s relentless brutality and to demonstrate that it is a credible advocate for the universal rights that are written into the U.N. Charter,” Obama said before the vote.

Chinese Ambassador Li Baodong said China had vetoed the resolution because of concerns it violated Syria’s sovereignty and territorial integrity.  China is the largest holder of U.S. Treasury bonds and the U.S. has little leverage over it. The Obama administration has been trying to pursue a softer policy with Russia which it calls a “reset.”

Syrian Ambassador Bashar Jaafari said the Assad regime is battling criminal elements and denied any massacre in Homs was occurring. He argued that Western powers were acting on ulterior motives to prop up Israel’s occupation of the West Bank.

After initially wavering in support of the Arab Spring when it erupted in Egypt last year, the Obama administration has more firmly sided with the protesters against dictatorships in the Arab world. American support was crucial for the overthrow of Egyptian President Hosni Mubarak and for the violent end of the dictatorship of Libyan Col. Moammar Gadafhi.

This story was originally posted at 10:30 a.m. and has been updated.