Syrian activists say more than 250 people, including a large number of civilians, were killed in fighting across Syria Thursday.

The London-based Syrian Observatory for Human Rights says it was the bloodiest single day in the 16-month uprising against President Bashar al-Assad.

Rebels seized control of major border posts with Iraq and Turkey Thursday, leading Iraqi officials to close off the border with Syria. Iraq also says the rebels killed at least 21 Syrian soldiers at the crossing.

Meanwhile, the United States says the U.N. Security Council has “utterly failed” after Russia and China vetoed a resolution threatening Syria with sanctions if it does not halt the violence.

The resolution would have extended the U.N. monitoring mission in Syria. It also threatened sanctions against the Assad government if it did not stop using heavy weapons against rebels and civilians within 10 days.