Ukraine’s military has begun what it is euphemistically calling an “anti-terror” operation against eastern Ukrainian provinces, vowing to crush the protesters who have taken over many government buildings in the cities of Donetsk and Luhansk.

Reports emerging from Kramatorsk, at the northern tip of the Donetsk Oblast have the first fighting taking place at an airfield, with shots fired on both sides and at least four of the protesters have been killed.

The details of what is happening in Kramatorsk remain unclear amid the chaos of a military offensive against a city of 200,000 people, but the Ukrainian military claims to have captured the airfield, and is looking to press those gains elsewhere in the city and in neighboring Slovyansk, which protesters have also seized.

Roads leading into Slovyansk are being guarded by checkpoints run by a militia loyal to the protesters, who have spent the past week preparing for the eventuality of a military offensive.

Ukraine’s military not only used ground troops, but also warplanes in the Kramatorsk Airfield battle. As-yet-unconfirmed reports claim that one of the Ukrainian planes, a SU-27, was shot down over the airfield.