The United States is concerned that the Syrian government is behind schedule in removing chemical weapons materials that are due to be destroyed under an international deal, US Defence Secretary Chuck Hagel has said.
The US has said that Syria must intensify its efforts to transport chemical weapons to the port of Latakia.
It comes after a report that less than 5% of the arsenal had been delivered and that work to destroy the weapons has fallen behind schedule.
"It is the Assad regime's responsibility to transport those chemicals to facilitate removal. We expect them to meet their obligation to do so," White House spokesman Jay Carney told reporters.
Meanwhile, a United Nations agency has delivered food to a rebel-held Damascus district, alleviating the plight of thousands of people trapped for months by a Syrian army siege.
The UN Relief and Works Agency (UNRWA), which cares for Palestinian refugees, said it had begun distributing 900 food parcels in Yarmouk camp, its biggest delivery there yet.
UNRWA spokesman Chris Gunness said the aid was the first to reach Yarmouk since 21 January when 138 food parcels were sent in.
Each parcel can feed a family of five for ten days, meaning the needs of the population still far outstrip aid deliveries.
"We hope to continue and increase substantially the amount of aid being delivered," said Mr Gunness.
He said: "With each passing hour their need increases."
Syria's state news agency SANA confirmed the aid delivery, saying Yarmouk's residents were "held hostage by armed terrorist groups" - its usual description of rebel forces.
UNRWA had blamed the authorities for preventing its convoy from reaching the neighbourhood on Sunday.
Two weeks earlier, aid convoys turned back after a government escort was fired on.
About 15 people are reported to have died from malnutrition in Yarmouk, originally an impoverished Palestinian refugee camp which now houses 18,000 Palestinians, as well as some Syrians.
Opposition activists say the government is using hunger as a weapon of war.
The Syrian government accuses rebels of firing on aid convoys and says it fears food and medicine will go to armed groups.
At peace talks between the two sides in Geneva, the UN is trying to negotiate passage for an aid convoy for 2,500 people also under siege in the Old City of Homs.
Gaining access for relief groups to reach an estimated 250,000 people trapped by fighting in Yarmouk, Homs and other areas is seen as a test for the peace talks.
The talks began last week and have not yet produced substantive results.
Syria's conflict began with popular protests against President Bashar al-Assad in March 2011, but evolved into a civil war after a crackdown by security forces led to an armed uprising.
More than 130,000 people have been killed and about six million have fled their homes.