The United States has condemned Bashar al-Assad's Syrian regime, accusing it of bombing starving civilians just hours after they received a long-delayed United Nations aid shipment.

Late on Thursday, the besieged rebel-held town of Daraya received its first United Nations food delivery since 2012, a lifeline for the suffering population.

Shortly afterwards, according to a witness and human rights monitors, Assad's forces bombarded the town, dropping indiscriminate barrel bombs from helicopters as residents shared food.

"The Syrian regime conducted multiple barrel bomb attacks on Daraya this morning," State Department spokesman Mark Toner said.

"That came just hours after the UN convoy arrived," he told reporters.

Toner said the fact that Assad had allowed the convoy in to the town at all was 'positive, but only a partial delivery and we would call for the rest of the supplies to be delivered as as soon as possible'.

And he said that only the United Nations should decide where and when to distribute aid inside Syria and that the fate of hungry populations must not be left to the regime.