Five civilians were killed and 10 others wounded Wednesday in airstrikes by the regime of Bashar Assad and Russia in northwestern Syria's Idlib, where a ceasefire is supposed to be in effect, the White Helmets civil defense agency said.

The airstrikes hit a hospital in the town of Tel Mannas, knocking it out of service and killing a medical staff.

Two people were killed in the town of Bashiriyya and one each in the towns of Tukh and Maarshiman in the Idlib de-escalation zone.

Activists reported several airstrikes on Idlib, the last major opposition stronghold in Syria, as regime forces captured new areas from insurgents.

An Assad regime military offensive began April 30 against the opposition in Idlib, home to 3 million people. More than half a million have been displaced by violence elsewhere.

Earlier this month, U.N. Secretary-General Antonio Guterres authorized an investigation into attacks on health facilities and schools in the opposition-held enclave, following a petition from Security Council members.

Last September, Turkey and Russia agreed to turn Idlib into a de-escalation zone where acts of aggression are expressly prohibited.

The regime and its allies, however, have consistently broken the terms of the ceasefire, launching frequent attacks inside the zone.

Syria has been locked in a vicious civil war since early 2011, when the Assad regime cracked down on pro-democracy protests with unexpected ferocity.

Since then, hundreds of thousands of people have been killed and more than 10 million others displaced, according to U.N. officials.