At least two people have been killed and nine wounded in gun battles between Ukrainian forces and pro-Kremlin militias that are threatening to scuttle international talks.

The clashes, which broke out on Sunday across the ex-Soviet state’s separatist eastern rust belt, came a day after masked gunmen stormed a series of police and security service buildings in co-ordinated raids that Kiev blamed on the “provocative activities of Russian special services”.

The heavily Russified region has been riven by unrest since a team of Western-backed leaders rose to power in February on the back of bloody protests against the old regime’s decision to reject an EU alliance and look for future assistance from the Kremlin.

Russia has since massed about 40,000 soldiers along Ukraine’s eastern frontier and threatened to halt its neighbour’s gas supplies over unpaid bills – a cutoff that would impact at least 18 EU nations and threaten further retaliation against the Kremlin.

Saturday’s attacks were especially unsettling for both Kiev and Western leaders because of their remarkable similarity to events leading up to Russia’s annexation of Ukraine’s Crimea peninsula last month.

The balaclava-clad gunmen were armed with special-issue assault rifles and scopes most often used by nations’ crack security troops.

Many wore unmarked camouflage uniforms similar to those seen on the highly trained units that seized the Black Sea peninsula in early March.

However, Russia denied any involvement. And it sternly warned Kiev late on Saturday that the use of force against pro-Russian protesters could ruin the chances of the two sides sitting down for US-EU mediated talks in Geneva on Thursday.

Ukraine’s Interior Minister Arsen Avakov announced the launch of an “anti-terrorist” operation in the eastern Donetsk region early on Sunday morning.

He said crack units from Ukraine’s SBU security service moved first into the city of Slavyansk to regain control of a police station seized by about 20 militants on Saturday.

But Avakov admitted his troops had to “regroup” after meeting stiff resistance and suffering casualties.

“There are dead and wounded on both sides. On our side – an SBU officer. The head of the SBU’s anti-terrorist centre has been wounded, as have four others,” Avakov wrote on his Facebook page.

“On side of the separatists – an unidentified number. The separatists have started to protect themselves using human shields.”

Russia’s state-run RIA Novosti news agency cited one local protester as saying a civilian had also been killed and two others injured.

The local administration separately reported a series of heavy clashes on a highway linking Slavyansk with the region’s capital Donetsk to the south.

The Donetsk administration said one person was killed and four wounded in an “ongoing armed standoff” on a stretch of the road connecting Slavyansk and the town of Artemivsk.

The statement added that authorities were still checking to see “whose side the casualties were on”.

Slavyansk residents meanwhile reported a run on stores and general panic among locals in the poor mining town of 100,000 people.

Saturday’s raids drew expressions of grave concern from world leaders and Russian warnings against any use of force against the militants.