GENEVA (Reuters) - The United States and Russia have not been cooperating to solve Syrian humanitarian problems recently, despite a ceasefire and 13 illegal sieges amounting to “strangulation of the civilian population”, a senior U.N. official said on Thursday.

Jan Egeland, the U.N. humanitarian adviser on Syria, said he hoped the two powers would join Turkey and Iran in pushing for breaking the “horrific gridlock” of aid convoys. They had all signaled in Thursday’s weekly meeting of a humanitarian task that they wanted improved humanitarian access, he said.

“‘Lift the sieges’ is our appeal. Sieges belong in the Middle Ages, they do not belong in 2017,” Egeland told reporters.

“A siege is not just a lawful military encirclement of military opponents - that’s legal. It is two elements that are illegal: one is the strangulation of the civilian population by denial of humanitarian aid, and the other one is lack of freedom of movement of civilians, including evacuation of wounded.”

The international community failed to lift a single siege by diplomatic means in 2016 but might yet do so in 2017, in talks in Astana, Geneva or elsewhere, Egeland said.

Russia, which backs President Bashar al-Assad’s government, and Turkey, which supports the rebels, are brokering talks in the Kazakh capital of Astana. At a first round of talks there in January, Russia, Turkey and Iran, another Assad ally, reaffirmed a shaky ceasefire between insurgents and the Syrian government.

That ceasefire has been repeatedly violated, with each side blaming the other. Meanwhile, fighting with jihadist groups such as Islamic State that are not included in the truce has gone on.

“We were great when there was co-leadership of the United States and Russia last year. We have not seen that of late,” Egeland said. “I am convinced it will come back.”

The U.N. has an aid convoy ready to go to al-Waer, a besieged suburb of Homs, and it could also supply other besieged areas in the coming days, which would be an important sign of goodwill for the political negotiations due in Geneva, he said.

Al-Waer would be a test case for other sieges, such as the towns of Foua, Kefraya, Madaya and Zabadani. They need aid soon to prevent a repeat of the starvation seen at the start of 2016 and to arrange evacuation of 80 sick people. Five had died in the last few days for want of evacuation, he said.