Syrian regime forces bolstered by Russian air strikes are making progress in the battleground city of Aleppo, rolling back the short-lived gains of a rebel offensive.

A watchdog voiced concerns meanwhile over reports of a chlorine gas attack Tuesday in nearby Saraqeb, close to where Russia said one of its helicopters was shot down the day before, killing the five people on board.

The Hague-based Organisation for the Prohibition of Chemical Weapons said in it "continues to examine any credible reports" of such attacks in the war-ravaged country.

It came as Moscow alleged a "poisonous agent" was fired Tuesday on the Salaheddin district of Aleppo from an area controlled by Islamists of the Nureddin al-Zenki rebel group, killing seven people.

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.

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.

Rebel fighters and their jihadist 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 forces have mounted a fierce fightback, retaking several positions from beleaguered rebels, a monitor said Wednesday.

The Syrian Observatory for Human Rights said government forces seized two hilltops and two small villages in the southwest 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."

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

The groups waging the offensive -- including 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 the southwestern district of Ramussa in a bid to cut off government forces and open a new route into the city for rebels.

- 'Setbacks' for rebels -

Footage obtained by AFP late Tuesday showed dozens of fighters carrying semi-automatic weapons and firing off several rockets reportedly at the front line in Ramussa.

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

They had managed to keep control of at least four hilltops and one small village.

Longtime regime ally Moscow launched an air campaign in support of President Bashar al-Assad 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 army's successes, especially in Aleppo, promises a great victory," the chairman of the Iranian parliament's national security and foreign policy committee, Alaedin Boroujerdi, said during a visit to Damascus Wednesday.

Iran is Assad's main regional backer and has provided steady military, financial, and political support to the regime.

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.

- 'Deeply flawed' plan -

On Tuesday, 35 NGOs including Save the Children and Oxfam said the Russian initiative for corridors out of Aleppo's rebel-held districts was "deeply flawed" and 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, the Observatory said.

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

A doctor from Idlib province told AFP on Wednesday that those affected had been treated and sent home.

The United States said it was formally investigating an air strike that allegedly killed 15 civilians near the IS stronghold of Manbij, the second such full probe it has launched.

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.