UNITED NATIONS — A United Nations Security Council effort to end the violence in Syria collapsed in acrimony with a double veto by Russia and China on Saturday, hours after the Syrian military attacked the city of Homs in what opposition leaders described as the deadliest government assault in the nearly 11-month uprising.

The veto and the mounting violence underlined the dynamics shaping what is proving to be the Arab world’s bloodiest revolt: diplomatic stalemate and failure as Syria plunges deeper into what many are already calling a civil war. Diplomats have lamented their lack of options in pressuring the Syrian government, and even some Syrian dissidents worry about what the growing confrontation will mean for a country reeling from bloodshed and hardship.

The veto is almost sure to embolden the government of President Bashar al-Assad, which brazenly carried out the assault on Homs on the day that the Security Council had planned to vote. It came, too, around the anniversary of its crackdown in 1982 on another Syrian city, Hama, by Mr. Assad’s father, Hafez, in which at least 10,000 people were killed in one of the bloodiest episodes in modern Arab history.

“It’s quite clear — this is a license to do more of the same and worse,” said Peter Harling, an expert on Syria at the International Crisis Group. “The regime will take it for granted that it can escalate further. We’re entering a new phase that will be far more violent still than what we’ve seen now.”