At least 20 people were killed and scores wounded in twin suicide and car bomb blasts in a Damascus suburb that contains Syria's holiest Shia Muslim shrine, state television said.

The so-called Islamic State has claimed responsibility for the attack, the IS-affiliated Amaq news agency said.

A Shia religious cleric on the state-run Ikhbariyah TV channel said the car bomb blast was not far from the shrine area in the main al-Taen street.

US condemns attack on besieged Syrian town by Assad regime

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 UN 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, President 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," US State Department spokesman Mark Toner said.

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

"Obviously such attacks are unacceptable in any circumstance but in this case they also hampered the delivery and distribution of badly needed assistance."

Mr Toner said the fact that President 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 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.

Meanwhile, a separate inter-agency convoy entered Douma in rebel-held Eastern Ghouta near Damascus last night.

UN mediator Staffan de Mistura, speaking to reporters on Thursday, said President Assad's government had approved UN land convoys to 15 of 17 government-besieged areas in June.

Air drops remain an option if the convoys did not move, he said.

Hussam Aala, Syria's ambassador to the UN in Geneva, said: "Discussions are still going on about one pending location. The rest were all approved."

Access to al Waer in Homs province was still under discussion, he said.

Around 1.9 tonnes of medicines for chronic diseases such as hypertension and diabetes as well as antibiotics, from the World Health Organization were on the delivery that entered Daraya.

However, the government did not approve delivery of three burns kits that would have been enough to treat about 30 people with dressings and pain killers, rejecting them from the approved list, WHO spokesman Tarik Jasarevic said.

There was also anger and frustration at the insufficient amount of food aid delivered, the Syrian Observatory for Human Rights reported. The British-based group tracks the war using sources on the ground.

It cited the Daraya local council as saying the supplies brought in would not last two weeks. The council said the population of Daraya is over 8,000 - more than double what the UN estimates.