A Syrian human rights group says more than 250 people, including a large number of civilians, were killed in fighting across Syria Thursday.

The figure was provided by the London-based Syrian Observatory for Human Rights.

It was one of the bloodiest days in the 16-month uprising against President Bashar al-Assad.

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

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

Ambassador Susan Rice called the vetoes dangerous and deplorable. She said the Council's failure to act is a recipe for more violence, terrorism, and a regional war.

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.

Russian Ambassador Vitaly Churkin says Moscow cannot accept any plan that threatens sanctions and leaves the door open to military intervention.

Ambassador Rice said the United States will work “with a diverse range of partners outside the Security Council” to pressure the Assad “regime” and bring aid to those in need in Syria.