Russian and Syrian armed forces will pause attacks on the Syrian city of Aleppo for eight hours on Thursday to allow civilians and rebels to leave the city, the Russian Defence Ministry said on Monday.

But Moscow ruled out a lasting ceasefire, a step that Western governments have been demanding, saying that would only give Islamist rebels in the city an opportunity to regroup.

The United Nations, which has long called for weekly 48-hour humanitarian pauses in the fighting, welcomed the Russian announcement.

"Any lessening of the violence, lessening of the fighting, any pause that's actually implemented, would be very much welcome," U.N. spokesman Stephane Dujarric told reporters.

"We will use whatever pause we have to do whatever we can. Obviously there is a need for a longer pause in order to get (aid) trucks in," he said.

Russian aircraft and Syrian forces loyal to President Bashar al-Assad have been bombarding rebel-controlled districts of Aleppo since the collapse of an internationally-brokered ceasefire last month.

The United States and some European states have accused Moscow and Syria of committing atrocities, but Russian officials deny that and accuse the West of abetting terrorists.

Speaking at a briefing in Moscow, Lieutenant-General Sergei Rudskoy, a senior Defence Ministry official, said rebels in parts of Aleppo were killing civilians while Western governments turned a blind eye.

"Given the situation, a unilateral ceasefire makes no sense, since Jabhat al-Nusra and groups allied to it will once again be given a breather, will regroup and restore their military capability," Rudskoy said, referring to a rebel group previously allied to al Qaeda.

He said Russia was working with other powers to achieve a peace deal for Aleppo, but that would take time, so in the meantime it had decided to initiate a humanitarian pause.

The pause is intended "first and foremost so that civilians can move freely, for the evacuation of the sick and wounded, and also for the removal of rebels," he said.

"On Oct. 20 from 0800 (1 a.m. ET) until 1600, a humanitarian pause will be implemented in the area of Aleppo. For that period, Russia's air force and Syrian government forces will halt air strikes and firing from other weapons," he said.