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 Lieutenant General Sergei Rudskoi. It had been due to expire Friday.

The lull had been greeted with high hopes by UN 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.

Jens Laerke, a spokesman for the UN’s humanitarian aid agency, described an ‘‘astronomically difficult situation,’’ although he declined to specify who was responsible.

Another UN official said Syrian opposition fighters were blocking the evacuations because the Syrian government and Russia were impeding deliveries of medical and humanitarian supplies into Aleppo.

The official said efforts were under way in Damascus, Aleppo, Geneva, and Gaziantep, Turkey, to try to move forward on the evacuations.