Aid convoys have arrived in four Syrian towns, but UN humanitarian adviser Jan Egeland said the government was still stopping aid getting to six others under siege.
"It is a violation of international law to prevent us from going," he said.
"I cannot understand why we cannot go there. Even wounded fighters have a right to be treated under international law."
He said sieges were blocking provision of medical care for needy people.
"We need the government of Syria really to help us in the medical area. Why not get in the nutritional stuff, vitamins and antibiotics, and doctors and nurses to these places? I hope there would be a real breakthrough on this in our contacts with the government," he told reporters on Thursday.
Egeland, speaking after hosting a regular meeting of countries involved in the Syrian peace process, said many innocent civilians were still in detention, including children.
He called on the United States, Russia and other countries to help get them released.
Egeland said aid was being delivered to the towns of Zabadani, Foua, Kefraya and Madaya on Thursday after two delays in the last week because of fighting.
The International Committee of the Red Cross and UN spokesman Jens Laerke later confirmed that the joint convoys had entered the four towns carrying vital supplies.
The Syrian government needed to give an answer within seven days on permission for convoys to go to six more areas, which its forces are still besieging almost three weeks into a truce, Egeland said.
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