A series of airstrikes in an insurgent-held Syrian province on Sunday killed at least 27 people, including four children, activists said.

The airstrikes in the northwestern Idlib province struck a crowded market and an apartment block, and came despite reports that a local truce between the government and insurgents had been renewed.

The Britain-based Syrian Observatory for Human Rights, which relies on a network of activists on the ground, said at least 21 people were killed by several airstrikes in Idlib city, including one that hit a popular market.

The airstrikes struck a crowded market and an apartment block (Reuters)

The group said another airstrike in the city of Maaret al-Numan, also in Idlib province, hit an apartment building, bringing it down and killing six people, including a mother and her four children who were trapped under the rubble.

The Local Coordination Committees, another activist-run group, and the Syrian Civil Defense, first responders who work in rebel-held areas, said the airstrikes in Idlib city killed at least 30 people and wounded dozens.

Both groups said the airstrikes are believed to have been carried out by Russia, which launched an aerial campaign in September to help shore up President Bashar Assad's forces. Russia denies targeting civilians.

Idlib is controlled by a coalition of Syrian rebel groups that includes al-Qaida's local branch, the Nusra Front.