Desperately needed aid has reached only 30 per cent of Syrians living in besieged areas and less than 10 per cent in hard-to-reach areas this year, even with the recent ceasefire, the UN humanitarian chief said Wednesday.

Stephen O'Brien told the Security Council that many of the 4.6 million Syrians in need 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 there is "a glimmer of hope," citing far fewer civilians killed and injured and progress on humanitarian access.

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

Since the beginning of the year, he said, convoys have reached 150,000 people in 11 of the 18 besieged areas in Syria.

But O'Brien said Syria's government still hasn't approved aid for three besieged areas "mere minutes' drive away from UN warehouses in Damascus" -- Duma, East Harasta and Daraya.

"The situation is dreadful in these areas, particularly in Daraya, where we continue to receive reports of severe shortages of food, clean water, medicines, electricity and basic commodities, with the food security and nutrition status thought to be disastrous, with even reports of people forced to eat grass," O'Brien said.

He also said he is "deeply troubled" that more than 210,000 civilians in the northern rural Homs towns of Rastan, Talbiseh, Houla, Termallah and Taldo, as well as 15,000 people in adjacent Habarnafse and surrounding communities in rural Hama, can't move in or out and have diminished access to food, clean water and medical care.

He strongly criticized Syrian authorities for barring or removing medical supplies from aid convoys, saying over 80,000 medical items have been removed. Those include treatments for child malnutrition and medicine for preventing bleeding after childbirth, which he called "scandalous."