US Defence Secretary Chuck Hagel on Thursday voiced concern over Syria's delay in handing over its chemical weapons arsenal and pressed Damascus to solve the problem.
"I do not know what the Syrian government's motives are -- if this is incompetence -- or why they are behind in delivering these materials," Hagel told reporters in Warsaw, adding that "they need to fix this."
The world's chemical watchdog said Wednesday that Damascus had handed over less than five percent of the most dangerous chemicals in its armoury.
Just two shipments of around 16 metric tonnes each of so-called Category 1 chemicals have left Syria's port of Latakia this month as part of an internationally backed disarmament plan supervised by the UN and the Organisation for the Prohibition of Chemical Weapons (OPCW).
"The United States is concerned that the Syrian government is behind in delivering the chemical weapons on time," Hagel said, speaking alongside Polish Defence Minister Tomasz Siemoniak.
"The Syrian government has to take responsibility to respect the commitment that had been made."
Hagel said he raised the issue with Russian Defence Minister Sergei Shoigu on Wednesday, asking him "to do what he could to influence the Syrian government".
In Washington, the White House also called on the Syrian regime to make good on its commitment to hand over its chemical weapons.
"It is the Assad regime's responsibility to transport those chemicals safely to facilitate their removal," said White House spokesman Jay Carney.
"We expect them to meet their obligations to do so".
The UN Security Council backed a US-Russian deal last year to eliminate Syria's vast chemical arsenal.
The agreement was brokered as a way to avert US missile strikes that Washington threatened after a chemical attack near Damascus that the US and other Western governments blamed on the regime.
Under the agreement, Syria's entire chemical arsenal is supposed to be eliminated by June 30.
Syria has declared around 700 tonnes of its most-dangerous chemicals and 500 tonnes of less dangerous precursor chemicals, which only become toxic when mixed with other compounds.
Almost all the chemicals and precursors, except for isopropanol which is to be destroyed within the war-torn country, are supposed to be removed by February 5.