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Fewer UN relief convoys reached starving civilians in Syria's besieged areas in March compared with February, prompting the United States to call for renewed pressure on the Damascus regime to allow access for aid deliveries.

The UN Security Council heard during a closed-door meeting that food aid in March reached 21 percent of Syria's 500,000 people trapped in areas under siege, down from 25 percent in February.

"We need the trend lines going in a much different direction," US Ambassador Samantha Power told reporters after the briefing.

"Every besieged area needs to be reached. All those in hard-to-reach areas need to be reached, and we are a long way from that."

The United Nations began scaling up deliveries of aid in Syria after a limited ceasefire went into force in February, hoping the desperately needed supplies would shore up a fragile peace process that began that month.

Aid deliveries to hard-to-reach areas - not totally under siege but where access remains restricted due to ongoing violence or its aftermath - climbed to 83,000 in March from 53,000 in February, according to UN aid officials.

Power singled out the rebel-held town of Daraya, which she said had received "not one crumb of UN food, since 2012" and was in urgent need of supplies after shelling polluted the local water table.

The United States and its partners have "appealed to those with influence" over the Syrian regime to press for more humanitarian access, said Power, in a thinly veiled reference to Russia.

Chinese Ambassador Liu Jieyi, the council's president for this month, said Daraya was "part of a bigger problem" of humanitarian access in Syria.

Nearly half a million Syrians are trapped in towns besieged by the Syrian army, armed groups or Islamic State group militants, with the complex war now in its sixth year

"We need to really look at the problem from all dimensions and, whoever can have influence to bear on the different parties, we should really work together," he said.

Nearly half a million Syrians are trapped in towns besieged by the Syrian army, armed groups or Islamic State group militants, with the complex war now in its sixth year.

The United Nations has asked Syria to grant access to 11 priority areas in April, but the government has so far approved only six, said Amanda Pitt, spokeswoman for UN humanitarian affairs.

Daraya, Douma and East Harasta have been left off the government's list.

An airdrop of humanitarian aid is planned for Deir az-Zour, which is under siege by IS militants, after a first delivery failed to meet its target.