The United Nations humanitarian chief says the U.N. and its partners are "ready to roll" with aid to Syrians in besieged areas but the deteriorating security situation in the country is keeping it from being delivered.

Undersecretary-General Stephen O'Brien said Tuesday that attacks have left nearly 2 million Syrians without access to electricity and the public water system, while food and medicine stocks are running dangerously low.

"We have supplies ready to roll, food rations, hospital supplies, ambulances, fuel for generators, water supplies and more," O'Brien said after briefing the Security Council. "We could deliver these within 24 to 48 hours if we have safe access."

He said Security Council members seemed receptive to his call for a weekly 48-hour humanitarian pause but the details must still be worked out.