-Kalashnikov-wielding gunmen on Saturday seized a police station and a security building in Ukraine's restive eastern industrial heartland amid spreading protests to press for the heavily Russified region to join Kremlin rule.

The coordinated attacks and a subsequent raid by a few hundred pro-Russian protesters on the police headquarters of the local capital Donetsk underscored the volatility of the crisis ahead of peace talks between EU and US diplomats and their Moscow and Kiev counterparts in Geneva on Thursday.

Ukraine's interim leaders have been facing unceasing pressure from Russia since their February ouster of an unpopular Kremlin-backed president and decision to seek closer ties with the West.

Moscow has massed tens of thousands of troops on Ukraine's eastern border after annexing its Black Sea peninsula of Crimea and nearly doubled the rates it charges Kiev for gas.

Russia is now ready to demand prepayment from the cash-strapped government for future gas deliveries or halt supplies -- a cutoff that would impact at least 18 EU countries and add further urgency to the worst East-West standoff since the Cold War.

But the seizures more immediately highlight how little sway Kiev's untested leaders have over pro-Russians who have since April 6 also controlled the Donetsk government seat and a state security building in the nearby eastern city of Lugansk.

Ukrainian Prime Minister Arseniy Yatsenyuk visited the region on Friday in a failed bid to pacify protesters with a vague promise of more sweeping local rights.