The United States urged both Syria and Russia on Friday to ensure that the remaining stockpile of Syrian chemical weapons is handed over to UN inspectors for destruction.

"We still continue to believe that the Assad regime can and must begin to take the necessary steps, including the packaging and destruction of certain materials on the site to demonstrate it is determined to fulfill its obligation," State Department spokeswoman Jen Psaki said.

The UN's Organization for the Prohibition of Chemical Weapons (OPCW), which is overseeing the elimination of Syria's toxic arms, has said some 92 percent of the declared stockpile has been removed from the country or destroyed.

But UN Special Coordinator Sigrid Kaag said Thursday that dangerous conditions on the ground have made it impossible to access the remaining chemical weapons containers.

Psaki stressed, however, that the remaining eight percent was in territory held by Syrian President Bashar al-Assad's regime.

"We need to continue to look for ways to get there regardless, that the regime has the responsibility to remove these weapons," Psaki told reporters.

Washington remains skeptical as to whether Assad has revealed the full extent of his country's stockpile.

"We have never taken the Assad regime at its word... and we continue to approach this process with our eyes wide open," Psaki said.

There are concerns that the regime may have used chlorine gas in an April attack. Chlorine, which has may uses, was not among the chemicals which the OPCW had required Damascus to declare.

The chemical weapons issue was raised during a phone call between US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov, who struck a deal in September to rid Syria of its chemical weapons.

The two nations reached the agreement, which sets a June 30 deadline for the destruction of the whole stockpile, after a sarin gas attack in the Damascus suburbs in August killed hundreds of people.

"Let's not forget that we've now removed 92 percent of the 100 percent of the declared" arms, Psaki said.

"That is a significant step forward. Does more work need to be done? Yes. But these are chemical weapons that the Assad regime will never again be able to use against their own people."