"The Ukrainian authorities consider the events of the day as a display of external aggression from Russia," Interior Minister Arsen Avakov said in a statement. "Units of the interior and defense ministries are implementing an operational response plan," he added.

On Saturday, men in the uniforms of Ukraine's now-defunct riot police Berkut occupied police headquarters in Donetsk, the eastern city that is one of the flashpoints of pro-Russia protests.

Pro-Russian activists carrying automatic weapons seized government buildings in Slovyansk, another eastern city, and set up barricades on the outskirts of the city.

Unknown men had opened fire on a police station in Kramatorsk, a town near Slovyansk, and police were engaged in a gunfight with them, according to a Facebook post by Avakov on Saturday evening. Reuters news agency later reported that 20 pro-Russian militants took control of the building.

Government buildings in the cities of Donetsk and Luhansk are still occupied by pro-Moscow protesters who have called on Russian President Vladimir Putin to send troops into eastern Ukraine.

The recent unrest also prompted the White House to announce that US Vice President Joe Biden would visit Kyiv on April 22.

"The vice president will discuss the latest developments in eastern Ukraine, where pro-Russian separatists, apparently with the support of Moscow, continue an orchestrated campaign of incitement and sabotage to destabilize the Ukrainian state," the White House said in a statement on Saturday.

On Wednesday, Ukrainian authorities set a 48-hour deadline for the separatists to end their occupation, but with that ultimatum having expired on Friday there has been no sign of action by police to force them out.

During an unannounced visit to Donetsk on Friday, acting Ukrainian premier Arseniy Yatsenyuk promised to grant more powers to the country's regions and that Kyiv would not clamp down on the east's right to use the Russian language.

Moscow has denied any suggestions that it is preparing to send in forces or split Ukraine, but the pro-Western government in Kyiv believes Russia is trying to look for ways to interfere. NATO says Russian armed forces are massing on Ukraine's eastern border, while Moscow says they are on normal maneuvers.

Kyiv has, however, fully rejected calls for an independence referendum that pro-Russia demonstrators have said should coincide with Ukraine's snap presidential polls scheduled for May 25.

Last month, Russia annexed Crimea after a referendum on the Black Sea peninsula. Ukraine, the EU and the US have condemned the referendum as illegal.

Ukraine has been in political and economic turmoil since February, when former President Viktor Yanukovych, a Kremlin ally, fled the country in the face of mass pro-Western protests against his rule.

ng/kms (AP, Reuters, AFP)