Syrian and Russian jets are stepping up strikes on opposition forces in northern Syria in retaliation for the sudden offensive that has cost the regime control of the country’s second largest city, Aleppo.

At least 25 people were killed in northwestern Syria in air strikes carried out by the Syrian government and Russia, the Syrian opposition-run rescue service known as the White Helmets said early on Monday.

Russian and Syrian fighter jets have struck the rebel-held city of Idlib on Dec. 1 amid attempts to push back an insurgency that has taken over most of Aleppo, Reuters reported, citing Syrian army sources.