A Russian soldier has shot dead a Ukrainian naval officer in eastern Crimea, Ukraine's Defence Ministry said on Monday, in one of few fatalities reported since Russia took control of the Black Sea peninsula.
The ministry gave few details of the death late on Sunday in the town of Novofyodorovka. Russian forces took over Crimea in an almost bloodless operation before Moscow annexed the region last month.
Pro-Russian protesters seized state buildings in three east Ukrainian cities on Sunday, triggering accusations from the pro-European government in Kiev that President Vladimir Putin was orchestrating "separatist disorder".
The protesters stormed regional government buildings in the industrial hub of Donetsk and security service offices in nearby Luhansk, waving Russian flags and demanding a Crimea-style referendum on joining Russia.
Protesters also later seized the regional administrative building in Kharkiv, Ukraine's second largest city, Interfax news agency reported.
All three cities lie close to Ukraine's border with Russia.
Interior Minister Arsen Avakov vowed to restore order in eastern Ukraine without using violence and also accused Ukraine's ousted president Viktor Yanukovich, whose political base was in Donetsk, of conspiring with Putin to fuel tensions.
"Putin and Yanukovich ordered and paid for the latest wave of separatist disorder in the east of the country. The people who have gathered are not many but they are very aggressive," Avakov said in a statement on his Facebook page.
"The situation will come back under control without bloodshed. That is the order to law enforcement officers, it's true. But the truth is that no one will peacefully tolerate the lawlessness of provocateurs."
Acting President Oleksander Turchinov called an emergency meeting of security chiefs in Kiev and took personal control of the situation, the parliamentary press service said.
Mainly Russian-speaking eastern Ukraine has seen a sharp rise in tensions since Yanukovich's overthrow in February and the advent of an interim government in Kiev that backs closer ties with the European Union.
Russia has branded the new government illegitimate and has annexed Ukraine's Crimea region, citing threats to its Russian-speaking majority - a move that has sparked the biggest standoff between Moscow and the West since the end of the Cold War.
Around 1,500 people protested in Donetsk on Sunday before breaking into the regional administration building, where they hung a Russian flag from a second-floor balcony, a Reuters witness said.
Protesters outside cheered and chanted "Russia! Russia!".
In the Luhansk protest, Ukrainian television said three people had been injured. Police could not confirm the report.
Talking to the crowd over a loudspeaker, protest leaders in Donetsk said they wanted regional lawmakers to convene an emergency meeting to discuss a vote on joining Russia like the one in Ukraine's Crimea region that led to its annexation.
"Deputies of the regional council should convene before midnight and take the decision to carry out a referendum," one of the protest leaders said, without identifying himself.
A local Internet portal streamed footage from the seized building, showing people entering and exiting freely. Soviet-era music was being played over loudspeakers outside.
The building houses the offices of Serhiy Taruta, a steel baron recently appointed by the interim government in Kiev as governor of a region with close economic and historical ties to Russia.
"Around 1,000 people took part (in the storming of the building), mostly young people with their faces covered," said Ihor Dyomin, a spokesman for Donetsk local police.
"Around 100 people are now inside the building and are barricading the building," he added.
In Luhansk, Reuters television showed images of hundreds of people outside the state security services building and a policeman in riot armour being carried away on a stretcher.
Ukrainian television said the Luhansk protesters were demanding the release of people detained by security services in recent days as well as a referendum on joining Russia.
"We don't want to join the EU, we don't want to join NATO. We want our children to live in peace," an unnamed woman told Ukraine's Channel Five in Luhansk.
Ukraine's state security services said on Saturday they had detained 15 people in Luhansk suspected of planning to overthrow the authorities and had confiscated hundreds of rifles, grenades and petrol bombs.
Pro-Russian demonstrators have held rallies in recent weeks in several eastern Ukrainian cities, not far from a border where Moscow has assembled tens of thousands of troops.