BEIRUT — A cease-fire to allow wounded civilians and rebels to leave besieged parts of Aleppo has been extended into the weekend by Russia, but hoped-for medical evacuations didn’t materialize by Friday evening because of a lack of security guarantees, officials and residents said.

The dawn-to-dusk “humanitarian pause” that began Thursday will last into Saturday on the orders of President Vladimir Putin, said Lt. Gen. Sergei Rudskoi, speaking in Moscow. It had been due to expire Friday.

The lull had been greeted with high hopes by U.N. officials, and the Syrian government opened a new corridor for those wanting to flee the neighborhoods shattered by weeks of Russian and Syrian airstrikes.

But by Friday evening, no evacuations were seen along the corridor, reflecting the intractable nature of Syria’s civil war, now in its sixth year.

Jens Laerke, a spokesman for the U.N.’s humanitarian aid agency, described an “astronomically difficult situation.”

He told reporters in Geneva that the evacuations couldn’t begin “because the necessary conditions were not in place to ensure safe, secure and voluntary” movement of people.

A U.N. official told The Associated Press that Syrian opposition fighters were blocking the evacuations because the Syrian government and Russia were impeding deliveries of medical and humanitarian supplies into Aleppo.