Suspected Russian air strikes killed at least 46 people in opposition-held parts of Syria Sunday, a monitor said, as government forces advanced in fierce clashes with rebels in east Aleppo.

Syria's government is waging an offensive to recapture all of second city Aleppo, and it has so far captured more than 60 percent of eastern districts that fell to rebels in 2012.

In Idlib province, in northwest Syria, at least 26 civilians were killed in suspected Russian strikes on the town of Kafr Nabel, the Britain-based Syrian Observatory for Human Rights monitor said.

An eyewitness told AFP warplanes hit several places in the town, including a market.

The Observatory says it determines whose planes carry out raids according to their type, location, flight patterns and the munitions involved.

The group said 18 people were also killed in suspected Russian strikes on the town of Maaret al-Numan, where an AFP photographer saw rescue workers and residents trying to pull survivors from rubble at a market.

The monitor reported two additional deaths, one in an earlier strike on Maaret al-Numan and another in Al-Naqir, also in Idlib.

It said six civilians, four of them children, had been killed in a government barrel bomb attack on the town of Al-Tamanah in the same province.

Russia, a staunch ally of President Bashar al-Assad's government, began a military intervention in support of Damascus in September 2015.

Moscow says it is targeting "terrorists" and has dismissed reports of civilian casualties in its strikes.

- Army advances in Aleppo -

In east Aleppo, government forces advanced against rebels, taking three neighbourhoods and pushing into a fourth, state media and the Russian defence ministry said.

The army and allied forces are nearly three weeks into an operation to recapture all of the city, divided between regime and rebel forces since 2012.

Tens of thousands of civilians have fled the offensive, which has made steady gains and threatens to deal Syria's opposition its worst defeat in the five-year civil war.

State television said late Sunday the army had captured the districts of Karm al-Tahan and Myessar and advanced into the Qadi Askar neighbourhood.

The Russian defence ministry said regime forces had also taken the district of Karm al-Katurji.

Rebels are increasingly under pressure in the remaining southeastern districts they control.

State news agency SANA said the air force was dropping leaflets over rebel-held areas urging "militants to abandon their weapons and... allow civilians and the sick and wounded to leave".

Damascus says rebels are preventing civilians from leaving the east and trying to use them as human shields.

But tens of thousands of residents have fled the east as the army has advanced, with some heading south to remaining rebel territory and others going to areas under government or Kurdish control.

The east has been pounded with air strikes, barrel bombs and artillery fire.

At least 311 civilians, including 42 children, have been killed in east Aleppo since the government assault began, the Observatory says.

- Nothing but rubble -

Rebel fire on west Aleppo in the same period has killed 69 civilians, including 28 children, it says.

On Sunday, the bombardment of rebel districts was so fierce it shook buildings in the west as well as in the east, AFP correspondents on both sides said.

The Observatory said a woman and two children were killed in the eastern neighbourhood of Fardos in government artillery fire.

The latest assault has added to the massive destruction in east Aleppo, which has seen some of the worst violence in the conflict that began in March 2011 with anti-government protests.

The army has encouraged residents to return to recently recaptured neighbourhoods, but many who have ventured across to see their old homes have found nothing but rubble.

"This is all we found, this photo of my niece. It is precious to us, and we found a copy of the Koran, so we brought that too," said Um Yayha, 55.

The Russian defence ministry also said Syrian government forces took control of the town of Al-Tal outside Damascus after a local truce.

"More than 500 rebels from the Fateh al-Sham Front have left Al-Tal for Idlib over the last days with 1,500 family members," it said, referring to Al-Qaeda's former Syrian affiliate.

Despite international outcry over the conflict, successive attempts to end it have failed.

UN envoy Staffan de Mistura on Saturday urged a return to negotiations, warning of a "creeping, ongoing guerilla (war) and no reconstruction" unless peace talks resumed.