UN: Number of Syrians Under Siege Doubled in 6 Months, Now Nearly 1 Million

U.N. figures point to the Syrian government and Islamic State as main culprits.

"Civilians are being isolated, starved, bombed and denied medical attention and humanitarian assistance in order to force them to submit or flee," O'Brien said of the situation.

Calling it a deliberate tactic of cruelty, O'Brien said that many Syrian civilians are faced with the choice of either “starve, get bombed or surrender.”

About 275,000 people remain trapped in rebel-held areas of Aleppo, where Syrian government airstrikes in the past week have killed dozens and knocked out medical facilities.

Outside of Aleppo, more than 400,000 people are encircled by conflict around the capital of Damascus.