GENEVA (Reuters) - The United Nations cannot use Russia’s unilateral pause in the fighting in Aleppo to send aid into the city’s besieged eastern zone because it does not have the needed security guarantees, a U.N. spokesman said on Friday.

Rebel fighters pray inside a building in western Aleppo city, Syria November 3, 2016. REUTERS/Ammar Abdullah

Russia and the Syrian army told rebels in Aleppo on Wednesday to leave by Friday evening, signaling an extended moratorium on air strikes in the city.

The Russian Defence Ministry and the Syrian army said rebels would be allowed to exit the city unharmed and with their weapons between 0900 and 1900 local time on Nov. 4 via two special corridors. Rebels rejected the calls.

“Humanitarian operations cannot be contingent on political or military initiatives,” U.N. spokesman Jens Laerke said.

“What we really need in Aleppo is for life-saving assistance to get in, to be delivered inside eastern Aleppo, and it is the responsibility of all parties to take the necessary measures, including providing security conditions to enable this.”

Russia has made several offers to encourage people to leave the besieged eastern zone of eastern Aleppo, which the U.N. says contains 250,000-275,000 civilians and about 8,000 rebels.

The U.N. has repeatedly said it wants a halt to the fighting by all sides so it can evacuate the wounded and get aid in.

Syria’s political opposition has said Russia has repeatedly duped the international community by making peaceful noises while reinforcing and strengthening its military efforts.

A fleet of Russian warships, including its only aircraft carrier, is sailing toward Syrian waters. NATO expects them to be used against civilians in eastern Aleppo.

Asked if Russia’s latest offer was a window of opportunity for U.N. aid efforts, Laerke said he was “trying my best not to comment” on the Russian initiative.

“Anything that saves lives is of humanitarian interest and we are happy with that,” he said.

“We still do not have the security assurances that we need for sending aid to eastern Aleppo,” he added.

Jessy Chahine-Mankouche, spokeswoman for U.N. Syria envoy Staffan de Mistura, said the United Nations was also against the evacuation of civilians unless it was voluntary.

“Our position remains the call for a complete nationwide cessation of hostilities but of course anything that contributes to saving lives, we welcome it,” Chahine-Mankouche said.

“There is no military solution to the Syrian crisis. The nationwide cessation of hostilities must be revived. The Syrian air force must agree to ground its air force over Aleppo and extremist elements must be isolated,” she said.