Syrian government air strikes on two besieged, rebel-held areas of the central province of Homs killed at least 31 people, 13 of them children, a monitoring group said on Sunday.

Sixteen members of the same family were among 24 people killed in raids late Saturday and Sunday on the town of Talbisseh, the Syrian Observatory for Human Rights said, updating an earlier toll.

They included 12 children and three women, said the Britain-based monitoring group which has a wide network of sources inside Syria.

Talbisseh was one of Syria's first areas to fall under rebel government control, after the 2011 outbreak of a revolt against President Bashar al-Assad.

It has been under total army siege for two years, and near-daily bombardment.

In the Waer district on the outskirts of Homs -- the only area of Syria's third city still in rebel hands -- air strikes on Sunday killed seven people, including a child, the Observatory said.

Activists say army bombing of densely-populated Waer has escalated, marring hopes of a truce similar to those reached in other parts of the country.

The escalation came after 47 children were killed in an October 1 suicide attack at a school in a government-held area of Homs city.

"The head of the military security branch was changed" after the school attack, said Hassan Abul Zein, an activist in Waer who spoke to AFP via the Internet.

"The new head of the branch has launched a barbaric campaign against the neighbourhood... where the humanitarian situation is deplorable," he said.

Homs was once dubbed "the capital of the revolution" against Assad.

But government forces retook control of the whole of the rest of the city in May when rebel fighters withdrew from central districts under a UN-brokered deal that ended a punishing two-year siege.

Elsewhere in the war-ravaged country, 12 civilians from the same family, including two women, were killed in government barrel bomb strikes on the rebel town of Busra al-Sham in southern Syria's Daraa province.

Human Rights Watch has accused the government of defying a February UN Security Council resolution that ordered all sides in the war to stop the "indiscriminate use of barrel bombs and other weapons in populated areas".

Also on Sunday, rebel fire on a school in the northern city of Aleppo killed a child and a man, and wounded 26 other people.

Aleppo city has been divided into government and rebel areas ever since a major offensive by insurgents in July 2012.

Syria's war began as a peaceful protest movement demanding political change, but morphed into a civil war after Assad's regime unleashed a brutal crackdown against dissent.

More than 180,000 people have been killed since March 2011, and nearly half the population have been forced by violence to flee their homes.