UNITED NATIONS (Reuters) - The number of besieged Syrians has more than doubled over the past year to nearly one million, the United Nations aid chief told the Security Council on Monday, as the United States named 13 Syrian military commanders it accused of killing civilians.

A burnt tent for displaced people is pictured after airstrikes on the outskirts of the rebel-held town of Atareb in Aleppo province, Syria August 4, 2016. REUTERS/Ammar Abdullah

Some 850,000 of those people are besieged by government forces, the United Nations said, while the rest are trapped by Islamic State militants and other armed groups.

“There is nothing subtle or complicated about the practice of besiegement. Civilians are being isolated, starved, bombed and denied medical attention and humanitarian assistance in order to force them to submit or flee,” U.N. aid chief Stephen O’Brien told the 15-member council during a monthly briefing.

O’Brien again urged strong action from the Security Council to back its resolutions calling for an end to attacks on civilians, humanitarian aid access and a lifting of sieges.

“Without strong backing from each of you, red lines will be crossed again and again; international humanitarian law will be trampled on; war crimes will be committed. And until there is action by you, there will be no accountability,” O’Brien said.

The council has been divided over how to end the nearly six-year war, with Syrian ally Russia, backed by China, protecting President Bashar al-Assad from council action by vetoing several resolutions, including a bid to refer the situation in Syria to the International Criminal Court.

U.S. Ambassador to the United Nations, Samantha Power, named 13 Syrian military commanders she said had been involved in killing and injuring civilians since 2011 through air and ground assaults, and detaining and torturing civilians.

“The United States will not let those who have commanded units involved in these actions hide anonymously behind the facade of the Assad regime,” Power told the council.

“Those behind such attacks must know that we and the international community are watching their actions, documenting their abuses, and one day, they will be held accountable,” she said.

However, Russian Deputy U.N. Ambassador Vladimir Safronkov accused Power of being hypocritical by not naming militants for killing civilians, and criticized her for naming the Syrian military commanders.

“You forgot even about your own golden standard of the presumption of innocence,” Safronkov told the Security Council. “This is something that can only be decided by legal proceedings. This is something that is elementary.”