Syria's military announced Wednesday it was scaling back its bombardment of rebels in devastated Aleppo, in a surprise move nearly two weeks after declaring an all-out assault to capture the city.

Once Syria's commercial hub, Aleppo has been divided by rebel groups in the east and regime forces in the west since violence erupted there in 2012.

The military announced the offensive to capture the whole city on September 22, ushering in a ferocious bombing campaign on opposition-held quarters that a monitor says has killed 270 people, including 53 children.

But Wednesday it said the bombardment would be reduced "after the success of our armed forces in Aleppo and cutting off all terrorist supply routes into the eastern districts".

"The military command has decided to reduce the number of air strikes and artillery on terrorist positions to allow civilians that want to leave to reach safe areas," said a statement.

It was not immediately clear what was behind the move, or if Russian air strikes would also be reduced.

The announcement came as Russia's TASS news agency said Foreign Minister Sergei Lavrov and US counterpart John Kerry discussed Syria by phone on Wednesday.

Forces loyal to President Bashar al-Assad have been waging their offensive on the city with the backing of Russian air power.

But the onslaught has come under intense international scrutiny amid accusations it was indiscriminate and devastating civilian infrastructure.

Air strikes were still taking place on Wednesday but were focused on the southern edges of Aleppo city, according to Rami Abdel Rahman of the UK-based Syrian Observatory for Human Rights.

"Syrian regime forces advanced from the city centre north into (rebel-held) Bustan al-Basha, and seized a large athletic complex there," Rahman said.

This was the first time the regime had entered the district since 2013 and there had been fierce clashes, he added.

On Monday, bombardment destroyed the largest hospital in rebel-held quarters, where an estimated 250,000 people live under government siege.

Hours later, Washington announced it would halt bilateral efforts with Moscow aimed at reviving a ceasefire, accusing Russia of trying to bomb civilians "into submission".

- 'Deeply shocking and shameful' -
But Moscow said Wednesday the "responsibility for the collapse of the truce lies with the US" given its decision to suspend contacts.

In another sign of increasing tensions, Russia said it was suspending joint research with the United States on nuclear energy projects.

Moscow and Washington's top diplomats had been working together since early this year to reach a solution to Syria's bloodshed, which has killed more than 300,000 people since 2011.

An agreement in September had envisioned an end to hostilities, increased aid deliveries, and eventual coordination between the two world powers against jihadists -- but it collapsed after a week.

Since then, France has stepped into the diplomatic vacuum with a draft UN resolution on a truce in Aleppo that it will submit to the Security Council this week.

French Foreign Minister Jean-Marc Ayrault will travel to Moscow on Thursday and Washington on Friday to try to garner support for the draft, his office said.

"I'm going to Moscow to give the view of France: this is unacceptable, it is deeply shocking and shameful", Ayrault said. "We must stop this massacre."

His ministry said the resolution calls for aid deliveries to the city's east, as well as the grounding of all Syrian and Russian planes in the area.

- Strike kills 19 civilians -
The United Nations concluded an air strike was responsible for the devastating attack on an aid convoy in northern Syria last month that killed nearly 20 people.

Lars Bromley, a researcher at the UN satellite collection and analysis agency, said his group's analysis "determined it was an air strike".

US officials have said Russian planes carried out the strikes on September 19 that hit the 31-truck convoy bringing aid to a town west of Aleppo.

Moscow has denied the accusation, and its military is carrying out its own investigation of the bombing, which destroyed 18 trucks and damaged a warehouse.

Russia and the US back opposing sides of Syria's war, with Washington lending support to some rebel groups and Moscow bolstering Assad both militarily and diplomatically.

Both countries are waging separate bombing campaigns against jihadist groups in Syria.

On Wednesday, unidentified raids on a northern village held by the Islamic State group killed 19 civilians including three children, according to the Observatory.

The monitor said it was unclear whether the strike on the village of Thalthana was carried out by the US-led coalition fighting IS, or Turkey, which is leading an operation against IS territory nearby.