The U.N. humanitarian chief says desperately needed aid has reached only 30 percent of Syrians living in besieged areas and less than 10 percent in hard-to-reach areas this year, even with the recent cease-fire.

Stephen O'Brien told the Security Council on Wednesday that many of the 4.6 million Syrians in these areas can't be reached because of insecurity and obstruction by combatants.

Since the cessation of hostilities came into effect one month ago, he said, far fewer civilians have been killed and injured and there has been progress on humanitarian access.

But O'Brien stressed that the U.N. and its partners are still "a long way from the sustained, unconditional and unimpeded access" required until international law and U.N. resolutions.