By Stephanie Nebehay

GENEVA (Reuters) - The United Nations vowed on Thursday to press ahead in securing medical evacuations of hundreds of sick and wounded from the Syrian city of Aleppo and demanded that the warring sides drop their conditions.

The United Nations aborted plans at the weekend to evacuate patients from rebel-held east Aleppo, which it had hoped to accomplish during a three-day lull in fighting last week, accusing all parties to the conflict of obstructing its efforts.

"We are not giving up," Jan Egeland, a U.N. humanitarian adviser, told reporters after the weekly meeting of the humanitarian task force, composed of major and regional powers.

"We had unanimous support from Russia, the United States and from all of the other countries in the room to try again. On all fronts," he said.

Egeland also said the Syrian government had rejected a U.N. request to deliver food and other aid supplies to rebel-held eastern Aleppo and an area in east Ghouta near Damascus as part of its plan for November.

"Which means that we need to overturn that decision," he added.

"It was very clear today that the Russians want to help us with the November plan implementation, would like to help us get access to east Aleppo," he said.

In all, the Assad government approved access to 23 of 25 areas sought by the U.N. next month, including 17 besieged areas, he said. But it will allow relief for only 70 percent of the more than 1 million people deemed in need, Egeland said. Surgical items are still not allowed in, "a notable exception".

"The war is getting worse, it's getting more ruthless and it's affecting more and more the children and the civilians," Egeland said.

In another sign that relations between Russia and the United States have frayed, Staffan de Mistura, the U.N. special envoy for Syria, announced that the two powers would no longer serve as co-chairs of a separate task force on the cessation of hostilities.

"...the cessation of hostilities is not really a major issue at the moment, but it needs to be kept alive," de Mistura said.

Volker Perthes, one of his senior advisers, would serve as acting co-chair of that task force, he said.

(Reporting by Stephanie Nebehay; Editing by Larry King)