Regime bombardment on Sunday killed 12 civilians, including three children, in an opposition bastion in northwestern Syria, a war monitor said. Idlib, a region of some 3 million people, has come under increasing fire by the regime and its ally Russia since late April, despite a months-old international truce deal. The opposition bastion, which is administered by Syria's former al-Qaida affiliate Hayat Tahrir al-Sham (HTS), includes most of Idlib province as well as slivers of the adjacent Aleppo, Hama and Latakia provinces. Seven civilians, including a young girl, were killed in regime artillery fire and airstrikes north of Hama province, the Syrian Observatory for Human Rights said.

Four more, including a man and his young son, lost their lives when regime aircraft struck near the town of Maaret al-Noman in Idlib province, the Britain-based monitor said.

A third child was killed in regime rocket fire on farmland in Idlib's village of Jadariya. On Friday and Saturday, strikes by the Syrian regime and its Russian ally killed 20 civilians, including six children.

More than 540 civilians have been killed in regime and Russian airstrikes on northwest Syria since the end of April, the Syrian Observatory says.

The United Nations says 25 health facilities in the region have been hit.

Being the opposition's last enclave, Idlib's prewar population of 1.5 million has swelled to around 3 million with new refugee waves after it was designated a "de-escalation zone" under the Astana agreement between Turkey, Russia and Iran in May 2017 to pave the way for a permanent political solution in Syria.

Turkey and Russia inked a deal for a buffer zone in September in Sochi to prevent a massive regime offensive on the Idlib region, near the Turkish border.

Following eight months of calm provided by the Sochi deal, the Assad regime intensified its attacks starting April 26 under the pretext of fighting HTS militants holed up in Idlib.

The intermittent attacks and bombardments have killed, wounded and displaced thousands. Residential areas have been destroyed by indiscriminate attacks, while numerous educational facilities, health facilities and residential areas have collapsed or have become unusable after being targeted by bombs.