Gunmen dressed in camouflage shot dead three Ukrainian policemen and wounded another in the restive eastern city of Donetsk on Thursday, the regional interior mistry said.
The bodies of the officers were found near the railway in the northwest of the industrial city, which is also the administrative centre of the separatist region of Donetsk.
The interior ministry said in a statement that the assailants had fled the scene and that a search and an investigation were under way.
Donetsk and the neighbouring border region of Lugansk were overrun by pro-Kremlin gunmen in early April and have remained under their effective control since.
Kiev in April launched an offensive aimed at regaining control of the vital provinces, which lie at the heart of Ukraine’s coal and steel industry.
Ensuing fighting has claimed more than 460 lives, and Ukrainian President Petro Poroshenko on Monday cancelled a 10-day ceasefire aimed at launching negotiations.
European mediators are trying to arrange a another truce between Kiev and Moscow, which denies backing the insurgency, and new talks are due to be held by Saturday.