Regime warplanes pounded Syria's second city Aleppo for a fourth straight day Wednesday in air raids that have killed at least 135 people, many of them children, a monitor said.

The attacks focused on rebel-held areas of eastern and northern Aleppo, once the country's commercial hub, the Syrian Observatory for Human Rights said.

Sunday's first day of the raids produced the highest daily toll with 76 dead in six neighbourhoods, among them 28 children, the Britain-based watchdog said.

Monday saw 20 dead including four children, and eight children were among 39 killed on Tuesday, said the Observatory which relies on a network of sources on the ground for its information.

It did not say whether or not any of those killed included rebels battling the regime of President Bashar al-Assad.

Many of the fatalities were caused by so-called "barrel bombs" packed with explosives and dropped from both fixed-wing aircraft and helicopters.

An estimated 126,000 people have been killed in Syria's civil war, which erupted after a brutal crackdown on pro-democracy protests first held in March 2011.

Doctors without Borders (MSF) reported more than 100 people killed by barrel bombs from helicopters, and already stretched hospitals overflowing with wounded.

Activists and the Observatory accuse Assad's forces of using barrels stuffed with concrete and TNT to cause the maximum number of casualties and damage.

A Syrian security source has denied that barrel bombs are used against what the regime calls "terrorists".

But another has said the military prefers such weapons instead of missiles because they are cheaper.

Most of the areas being targeted are in the east of the city and near regime-controlled parts of Aleppo province, one of the front lines in the conflict.

The opposition National Coalition has said the "systematic raids on Aleppo demonstrate the regime's rejection of a political solution".

A UN-backed peace conference dubbed Geneva 2 is due to be held in the Swiss city of Montreux on January 22.