The United States is working with Russia to improve access to besieged areas in Syria and to stop the Syrian government from removing medical supplies from aid convoys, a senior U.S. official said on Wednesday.

Antony Blinken, deputy U.S. Secretary of State, said that major and regional powers were monitoring a fragile cessation of hostilities that went into force on Saturday to "prevent any escalation" but it was a "challenging process".

"At the end of the day the best possible thing that could happen is for the cessation of hostilities to really take root, and to be sustained, for the humanitarian assistance to flow and then for the negotiations to start that lead to a political transition," Blinken told a news conference.

The World Health Organisation said Syrian officials had "rejected" medical supplies from being part of the latest convoy to the besieged town of Moadamiya on Monday. WHO spokesman Tarik Jasarevic said they included emergency kits, trauma and burn kits and antibiotics.

"We are indeed very concerned about reports that medical supplies were removed from some of the aid convoys. This is an issue that was brought before the task force," Blinken said, referring to the International Syria Support Group (ISSG).

"We are now working, including with Russia, to ensure that going forward medical supplies remain in the aid convoys as they deliver assistance."

Russian officials were not immediately available to comment.

"The removal of those supplies is yet another unconscionable act by the regime, but this is now before the task force and we will look in the days ahead as assistance continues to flow to make sure that those medical supplies are in fact included," Blinken said.

The humanitarian task force, chaired by Jan Egeland, meets again in Geneva on Thursday.

Another ISSG task force on the cessation of hostilities is handling reports of violations of the truce, which does not include Islamic State or the al Qaeda-linked Nusra Front.

"We're then able to immediately try to address them and to prevent them from reoccurring and thus to prevent any escalation that leads to the breakdown of the cessation of hostilities," Blinken said, after talks with U.N. Special Envoy Staffan de Mistura.

"That's the most effective way to try to keep it going and then to deepen it. But it is a very challenging process, it's fragile and we have our eyes wide open about those challenges."

By Reuters