Three surface-to-surface missiles fired by Syrian regime forces in the northern city of Aleppo have left 58 people dead, among them 36 children, a watchdog said on Sunday, updating an earlier toll.

The Syrian Observatory for Human Rights revised its toll upwards after more bodies of women and children were removed from the rubble in Aleppo's Tariq al-Bab district, which was struck by the powerful missiles on Friday.

Activists say the missiles were fired from army base 155 near Damascus, though their reports could not be independently verified.

A security official in Damascus told AFP late last year the Syrian regime has developed its own version of the Scud, while NATO has since reported the use of ballistic missiles in the country.

According to the Observatory, the army's use of the missiles is part of a bid to overrun neighbourhoods east of Aleppo, which are almost completely under rebel control.

On Friday, the opposition Syrian National Coalition vented fury at world powers following the missile strikes and announced a boycott of international meetings.

Coalition chief Ahmed Moaz al-Khatib said the boycott was "a message of protest to all governments of the world..." who were merely looking on as the Syrian people were being killed.

The group had been due to attend a Friends of Syria meeting in Rome next Thursday, while Khatib had also been invited to Moscow, and to the United States.

"We cannot visit any country until there is a clear decision on this savage, aggressive regime," Khatib said of President Bashar al-Assad's administration.

The Coalition singled out Damascus ally Russia, saying its leaders were "ethically and politically responsible because they continue to support the (Syrian) regime with weapons".

On Saturday, US State Department spokeswoman Victoria Nuland condemned the missile strikes, saying they were "the latest demonstrations of the Syrian regime's ruthlessness and its lack of compassion for the Syrian people it claims to represent."

At least 110 people were killed Saturday nationwide -- 46 civilians, 35 rebels and 29 soldiers, the Observatory said.

The almost two-year conflict has cost the lives of more than 70,000 people according to the UN.