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.

Syrian armed opposition forces have entered Aleppo just three days into their surprise offensive, marking the first time they have set foot in the country’s second largest city since government forces recaptured the city in 2016.