At least 23 people were killed on Sunday in an explosion during airstrikes in Syria’s Idlib province, the Syrian Observatory for Human Rights reported. The airstrikes are part of President Bashar al-Assad government’s attempt to regain control over the last bastions of the rebel-held region.

The observatory said it was unclear whether the explosion was triggered by the airstrike or a clash between jihadist and rebel groups. “Regime and Russian strikes are continuing today [Sunday] on several parts of Idlib province,” Rami Abdel Rahman, the head of the Britain-based monitoring organisation, told AFP.

Eastern Ghouta is another area that the rebels still have control over. State news agency, SANA, reported that the army had regained control of the military base there. “Units from the Syrian Arab Army have brought an end to the encirclement of the armoured vehicles base in Harasta,” AFP quoted SANA.

Russia had began a military intervention in September 2015 to help the Syrian government regain territory from the Islamic State group.