Syrian regime forces bolstered by Russian air strikes recaptured territory overnight in the southwest suburbs of the battleground city of Aleppo, rolling back the short-lived gains of a rebel offensive.

Rebels and their jihadists allies launched an assault Sunday in a bid to ease a more than two-week government siege of opposition-held districts of the city.

But regime fighters have put up a fierce fightback, retaking several positions from beleaguered rebel forces, a monitor said Wednesday.

The Syrian Observatory for Human Rights said government forces seized two hilltops and two small villages in the southwest suburbs of Aleppo late Tuesday.

"The regime is launching counter-attacks to absorb the fierce rebel offensive," said Observatory head Rami Abdel Rahman.

"The opposition offensive has not achieved the results that were expected at this stage," he said.

An AFP journalist in east Aleppo said clashes and shelling could be heard throughout the night, followed by barrel bomb attacks and air strikes in the early morning.

The groups waging the offensive -- including fighters from Al-Qaeda's former Syria affiliate and the powerful Islamist Ahrar al-Sham -- have promised to end the government encirclement of eastern parts of Aleppo.

They are seeking to capture Ramussa, a district in Aleppo's southwest suburbs, in a bid to cut off government forces and open a new route into the city for rebels.

- 'Long and gruesome battle' -

But they have struggled to hold newly-acquired territory in the face of heavy Russian air strikes, Abdel Rahman said.

They have managed to keep control of at least four hilltops and one small village, he added.

Longtime regime ally Moscow launched an air campaign in support of President Bashar al-Assad's forces in September.

Wednesday's edition of Al-Watan, a newspaper close to the government, said government forces, backed by Russian air strikes, "advanced again south and southwest of Aleppo causing major setbacks" for rebel factions.

And pro-regime website Al-Masdar News said an initial rebel advance into the Ramussa district was pushed back "following a long and gruesome battle".

The Britain-based Observatory said more than 50 rebels and allied jihadists had been killed since the assault began, as well as dozens of regime troops.

Overnight, at least 10 civilians, including four children, were killed in rebel shelling of government-controlled districts on Aleppo's southwestern edges, the monitor said.

More than 40 civilians have been killed by shelling on government-held neighbourhoods since Sunday.

The battle for Aleppo -- Syria's second city -- is critical for both the regime in Damascus and the forces seeking to overthrow it.

It was Syria's commercial hub until 2012, when clashes left it roughly divided between government troops in the west and rebels in the east.

- 'Deeply flawed' plan -

Eastern districts came under government siege on July 17, sparking concerns for the estimated 250,000 people still living there.

Last week Russia announced the opening of "humanitarian corridors" to allow residents and surrendering fighters to flee for government-held territory.

This was met with scepticism by residents and international observers, and 35 NGOs in a statement on Tuesday called the initiative "deeply flawed".

The groups, including Save the Children and Oxfam, urged implementation of a UN call for a weekly 48-hour humanitarian pause in Aleppo.

In the surrounding province, 24 people reportedly suffered breathing difficulties in Saraqeb, a town 50 kilometres (30 miles) south of Aleppo, after a barrel bomb attack on Tuesday, the Observatory said.

Residents said the attack had used chlorine gas, but the monitor could not confirm this.

The incident took place close to where Russia said on Monday one of its military helicopters was shot down, killing the five people on board.

Syria's conflict has killed more than 280,000 people and drawn in world powers on both sides since it erupted in March 2011.

Half the country's population has been forced to flee their homes with an estimated five million seeking refuge in neighbouring countries.