Syrian and Russian warplanes have launched a wave of air strikes in northern Syria, killing dozens in areas held by a rebel alliance battling to take control of second city Aleppo.

The air strikes, which began Saturday and continued Sunday, killed 45 civilians in and around Aleppo and 22 in neighbouring Idlib province, the Syrian Observatory for Human Rights said.

The raids came as the Islamist Faylaq al-Sham Islamist faction, part of the rebel alliance, said it had begun a new offensive "to liberate" the regime-held area of Zahra on Aleppo's western outskirts.

The Britain-based Observatory and opposition fighters said a car bomb exploded in Zahra Sunday, but did not mention casualties.

Late Sunday, at least 15 rebels were killed and 20 wounded in a suicide bombing near their bus in Idlib near the Atme border crossing with Turkey, the monitor said.

It said Sunday's air strikes targeted areas held by the Army of Conquest, an alliance of rebel, Islamist and jihadist forces that has mounted a major Aleppo offensive.

"The intensification of the strikes in Idlib is due to the fact that this province is the main source of fighters for the Army of Conquest," said Observatory head Rami Abdel Rahman.

An AFP correspondent in rebel-held eastern Aleppo said the strikes were especially intense around the southern district of Ramussa, seized by rebels earlier this month in a major setback for forces loyal to President Bashar al-Assad.

Nine other civilians were killed in rebel shelling of regime-held western Aleppo Saturday, the Observatory said.

Aleppo, Syria's former economic hub and a focal point of its five-year civil war, has been divided between a rebel-held east and regime-controlled west since mid-2012.

- Intensifying battle -

Fighting for the city has intensified after regime troops seized control of the last supply route into rebel-held areas in mid-July.

After a nearly three-week siege, rebels took Ramussa on August 6, linking up with opposition-held neighbourhoods.

The Army of Conquest then announced an ambitious bid to capture all of Aleppo, which if successful would be the biggest opposition victory yet in Syria's conflict.

Alliance fighters late Sunday began an assault southeast of Ramussa on an area containing a former cement plant turned into an army barracks, where a large amount of weapons and military vehicles are stored, the Observatory said.

The rebels "want to secure the supply line they opened" last week into rebel districts, Abdel Rahman said.

The increased fighting has raised concerns for the estimated 1.5 million civilians still in Aleppo, including some 250,000 in rebel-held areas.

The United Nations has called for regular 48-hour pauses in the fighting to allow aid into the city, which has suffered from severe shortages of food, water and medical supplies.

Russia began air strikes last September in support of Assad, helping the regime to consolidate its hold on loyal areas and regain some territory.

The defence ministry in Moscow said Sunday six long-range bombers from Russia had struck around Deir Ezzor, a stronghold of the Islamic State group.

IS controls large parts of Deir Ezzor city and most of the oil-rich eastern province of the same name -- part of the territory in Syria and Iraq that it seized in mid-2014.

- Girl evacuated -

Russia said the Tupolev bombers carried out raids southwest, east and northeast of the city, wiping out two command posts, six arms depots, IS vehicles and "a large number of fighters".

IS emerged amid the chaos of Syria's conflict, a complex and multi-front war that has killed more than 290,000 and displaced millions since beginning with anti-regime protests in March 2011.

A US-led coalition is also battling IS in Syria and Iraq, with air strikes and backing for the Syrian Democratic Forces, an Arab-Kurdish militia alliance, which on Friday took full control of the strategic city of Manbij after IS retreated.

The jihadists took some 2,000 civilians as they fled to serve as human shields. Hundreds were released on Saturday but the SDF said the fate of many remained unclear.

On Sunday, the SDF said they had established a military council to push IS fighters out of Al-Bab, the alliance's next target in the same province.

In rare good news, a 10-year-old girl shot in the besieged town of Madaya was evacuated to a Damascus hospital where she was in a stable condition, the Observatory and a security source said.

Ghina Quwayder's leg was shattered when she was shot by a government sniper in the southwestern town this month while buying medicine for her mother, Amnesty International said.