Syrian and Russian aircraft have launched intense air strikes on opposition strongholds in northern Syria to prevent rebels sending reinforcements to a crucial battle in Aleppo, a monitor said Thursday.

Air strikes on Idlib city, 60 kilometres (35 miles) southwest of Aleppo, killed 25 people including 15 civilians on Wednesday, according to the Britain-based Syrian Observatory for Human Rights.

Regime aircraft continued to pound rebel positions across Idlib province on Thursday as well as parts of Aleppo province, it said.

"Regime and Russian aircraft are carrying out dozens of raids every day on Idlib province and the west of Aleppo province to prevent reinforcements reaching rebel positions," said Observatory head Rami Abdel Rahman.

Idlib is dominated by the same alliance of rebels and jihadists that is fighting in Aleppo, including the former Al-Nusra Front, which has renamed itself Fateh al-Sham Front after renouncing its status as Al-Qaeda's Syrian affiliate.

Southern Aleppo has been the scene of intense fighting since July 31, when the "Army of Conquest" alliance launched a major offensive to break a regime siege of opposition-controlled districts in the city's east.

It took the southern district of Ramussa on August 6, linking up with opposition-held neighbourhoods. But neither side has achieved a decisive victory despite hundreds dead on both sides.

Each side is now trying to cut off the other's supply routes.

Despite its air power, the regime has been unable to gain headway in street-to-street battles, said Abdel Rahman.

"The rebels have put all their forces into this battle and regime forces have been exhausted," said Abdel Rahman.

Aleppo has been roughly split between opposition control in the east and government forces in the west since mid-2012.

Analysts say a decisive breakthrough by one side in Aleppo is unlikely for the time being.

On Thursday, the regime continued to pound the east of Aleppo city, where 146 civilians including 22 children have died in air strikes since 31 July.

More than 290,000 people have been killed and millions displaced since the beginning of Syria's civil war, which started in 2011 with anti-regime protests.