At least 28 civilians have been killed in air attacks in the opposition stronghold of Idlib province in northwestern Syria, according to a war-monitoring group.

Four children were among the dead in the overnight air raids on the town of Amanaz, a few miles from the Turkish border, the UK-based Syrian Observatory for Human Rights (SOHR) said on Saturday.

An AFP news agency correspondent who toured Armanaz on Saturday said entire apartment blocks had been flattened by the bombardment.

It was not immediately clear whether the attack was carried out by Syrian government warplanes or those of its ally Russia.

According to the SOHR - which gathers its information from a network of sources inside Syria - dozens of people are still missing after the bombardment.

READ MORE: Mapping out Syria's remaining battles

Secondary air raids were carried out as a search-and-rescue operation was under way for victims, the monitor said.

Anadolu news agency reported at least 40 civilians were killed and 70 others wounded in Amanaz.

The Russian and Syrian militaries say they only target 'insurgents' and deny killing civilians.

The surge in bombing raids has forced hospitals in the province to close, medical charity Doctors Without Borders said on Friday.

Syrian and Russian jets have intensified attacks in Idlib. The increased aerial bombardment began after rebels launched an offensive against government-held areas in the northwest of the country on September 19.

Last month, Russia, Turkey and Iran agreed to create a "de-escalation zone" in the province, as part of their efforts to establish a broad ceasefire in war-torn Syria.

Russian President Vladimir Putin and Turkish counterpart Recep Tayyip Erdogan agreed on Thursday to step up efforts to establish a safe zone in Idlib.

Syria said the Idlib deal does not cover hardline opposition groups such as the Islamic State of Iraq and the Levant (ISIL) group and Hayet Tahrir al-Sham.

Syria has been locked in a vicious civil war since early 2011 when the government of President Bashar al-Assad cracked down on pro-democracy protests.

Since then, hundreds of thousands of people have been killed and more than 10 million displaced, according to the UN.