Six extremists were killed Wednesday (May 27th) in an explosion that hit their arms depot in north-west Syria while Russian warplanes flew overhead, a war monitoring group said.

The Syrian Observatory for Human Rights did not say if Russia was behind the blast that killed the non-Syrian extremists allied with the Turkistan Islamic Party, a Uighur-dominated extremist group.

"We do not know if it is the result of aerial bombardment or an explosion inside the warehouse," Observatory head Rami Abdel Rahman said.

If confirmed, it would mark the first Russian strike on the Idlib region since a ceasefire went into effect in March.

Home to some three million people, the last major opposition bastion in Idlib is controlled by Tahrir al-Sham.

A Russian-backed regime offensive between December and March displaced nearly a million people in the north-west, but at least 140,000 have returned since a ceasefire went into effect, according to the UN.

The truce, which coincided with the novel coronavirus (COVID-19) crisis, had put a stop to the relentless airstrikes by the regime and Russia that killed at least 500 civilians in four months.

Wednesday's explosion hit an arms depot in the al-Taybat village near the town of Jisr al-Shughur, which is controlled by the Tahrir al-Sham allied Turkistan Islamic Party, the Observatory said.

Russian warplanes were flying overhead at the time of the blast, it added.

Nearly half of Idlib's population consists of Syrians displaced from other parts of the country.

The war in Syria has killed more than 380,000 people and displaced nearly half of the country's pre-war population since it started in 2011.