A pro-Russian protester burns tires in preparation for a battle with the Ukrainian special police force on the outskirts of the eastern Ukrainian city of Slavyansk on Sunday. Photo: AFP

Graphics: AFP/GT

Kiev plans to launch a "full-scale anti-terrorist operation" involving the army against pro-Russian separatists in the country's east, Ukraine's acting President Oleksandr Turchynov said on Sunday.

In a televised address to the nation, Turchynov said Russia was waging a war against Ukraine by sowing disorder in the east of the country, but he offered not to prosecute any militants who gave up their weapons by Monday morning.

The remarks came after Ukrainian security forces launched an "anti-terrorist operation" to clear pro-Russian gunmen from state buildings in the eastern city of Slavyansk.

Ukraine's Interior Minister Arsen Avakov said both sides had suffered casualties in the offensive in Slavyansk, where about 20 pro-Kremlin gunmen seized the police station and later occupied the city's SBU (Ukrainian Security Service) building on Saturday.

"There are dead and wounded on both sides," Avakov wrote on his Facebook page. According to him, a security officer was killed and five others were wounded.

One pro-Russian activist was also killed and another two were injured in the clashes, Russian news agency RIA reported, citing a local militant.

The latest wave of unrest began when protesters seized the seat of government in Donetsk, followed by similar actions in Lugansk and Kharkiv.

Separatist gunmen were in control of the police station in Kramatorsk after a gunfight on Saturday.

Protesters on Sunday also seized control of the mayor's office in the town of Mariupol on the Azov Sea, local paper Priazovsky Worker reported.

The West has expressed alarm that Russia is deliberately stoking tensions in order to justify a Crimea-style annexation.

Avakov said the events were seen in Kiev as an "act of aggression" by Russia, which has flatly denied any role in the unrest sweeping Ukraine's east.

Zhang Hong, a research fellow on Ukrainian studies at the Chinese Academy of Social Sciences, said as long as Kiev could effectively quell the unrest, Moscow is unlikely to intervene.

However, if it fails to restore stability in a timely fashion, which then results lasting chaos in eastern Ukraine, then the scenario in Crimea may be replayed, Zhang said.

US Secretary of State John Kerry on Saturday phoned his Russian counterpart Sergei Lavrov. Kerry "expressed strong concern that attacks today by armed militants in eastern Ukraine were orchestrated and synchronized, similar to previous attacks in eastern Ukraine and Crimea," said a senior State Department official, speaking on condition of anonymity.

"Militants were equipped with specialized Russian weapons and the same uniforms as those worn by the Russian forces that invaded Crimea. The secretary made clear that if Russia did not take steps to de-escalate in eastern Ukraine and move its troops back from Ukraine's border, there would be additional consequences," the official added.

US Ambassador to the UN Samantha Power said Sunday on ABC's This Week the US is prepared to step up sanctions against Russia if the recent actions in Ukraine continue.

A wave of US sanctions unveiled in March blacklisted officials and businesspeople close to Russian President Vladimir Putin to protest at Moscow's takeover of Crimea.

Zhang believed the new sanctions may target Russia's financial and energy sectors.

On Sunday, France also said it would support new sanctions against Moscow if there was a military escalation in Ukraine.

Meanwhile, the White House said Saturday that US Vice President Joe Biden would travel to Kiev next week to show support for the Ukrainian government. He will become the most senior US official to visit since the crisis began.

Xing Guangcheng, an expert on Russian studies at the Chinese Academy of Social Sciences, said that the tug-of-war between Moscow and the West is a major factor in the current crisis. However, deep-rooted problems in Ukraine's political realities, which have been seen in the changing regimes since the orange revolution in 2005, remain the cause for the escalating tensions. He warned that it would jeopardize the upcoming presidential polls on May 25.

Ukraine's embattled Prime Minister Arseniy Yatsenyuk promised during an unannounced visit to Donetsk on Friday to grant more powers to the country's regions and protect the east's right to use the Russian language. But gunmen want to hold independence referendums coinciding with the presidential polls.

Separately, Russia's foreign ministry Sunday said in a statement that Lavrov plans to "devote special attention to the situation in Ukraine" during his meetings with Chinese President Xi Jinping and foreign minister Wang Yi in Beijing on Tuesday.

"As is known, China in its approach sticks to a balanced and objective position, demonstrating understanding of the entire totality of factors, including historical ones, leading to new realities in this region," said the ministry.

Agencies contributed to this story