Two helicopters shot down killed one Ukrainian pilot

Interior Minister Arsen Avakov said one Ukrainian pilot had been killed, and other people wounded. Nine rebel checkpoints had been seized, he said.

However, separatists at three Sloviansk checkpoints told the BBC they were still in control there.

Pro-Russian gunmen have shot down at least two helicopters, reportedly capturing one pilot.

The fighting appears to be concentrating on the periphery of the city in the regional capital Donetsk, according to BBC.

Sloviansk is a stronghold for pro-Russian separatists who are exerting increasing control in the region.

On Thursday, Russia's foreign ministry warned that any assaults by Ukraine's troops in the region would have "catastrophic consequences", triggering fears of an invasion by Moscow.

In a statement on Friday , Mr Avakov said the "active phase" of the operation began at 04:30 local time (02:20 GMT).

Ukrainian government soldiers guard a checkpoint near Sloviansk. Photo: 1 May 2025

He said interior troops and the National Guard were involved in the operation in the Sloviansk-Kramatorsk region.

"The terrorists opened fire with heavy weapons against Ukrainian special units. A real battle with professional mercenaries is going on", Mr. Avakov said, adding that the separatists were using the tactics of hiding behind civilians in residential buildings.

Earlier, Ukraine's acting President Olexandr Turchynov reinstated military conscription. Mr. Turchynov said his forces were "helpless" to quell the unrest in some parts of the east, saying the goal was now to prevent it from spreading. He also said Ukraine was on "full combat alert" amid fears that Russian troops could invade.

Some 40,000 Russian troops are stationed near the Ukrainian border.