Pro-Russian rebels in the Ukraine's eastern city of Slavyansk freed seven European military observers on Saturday after holding them hostage for eight days, while Kiev pressed on with a military campaign to reclaim rebel-held territory in the area.
The riot in the Black Sea port of Odessa that ended in a deadly blaze in a trade union building was by far the worst incident in Ukraine since a February uprising that ended with a pro-Russian president fleeing the country. It also spread the violence from the eastern separatist heartland to an area far from the Russian frontier, raising the prospect of unrest sweeping more broadly across a country of around 45 million people the size of France.
The Kremlin, which has massed tens of thousands of soldiers on the eastern Ukrainian frontier and proclaims the right to invade to protect Russian speakers, said the provisional government in Kiev and its Western backers were responsible.
The Odessa bloodshed came on the same day as the biggest push yet by the government in Kiev to reassert its control over separatist areas in the east, hundreds of kilometres away, where heavily armed pro-Russian rebels have proclaimed a "People's Republic of Donetsk". The rebels there aim to hold a referendum on May 11 on secession from Ukraine, similar to one staged in March in Ukraine's Crimea region, which was seized and annexed by Russia in a move that overturned the post-Cold War diplomatic order.
Rebels in the eastern town of Slavyansk, their most heavily fortified redoubt, shot down two Ukrainian helicopters on Friday, killing two crew, and stalled an advance by Ukrainian troops in armoured vehicles. On Saturday the government said it was pressing on with the offensive in the area for a second day.
The capture of the military monitors, on a mission from the Organization for Security and Cooperation in Europe (OSCE), was a major diplomatic issue for the West and their release could help relieve pressure on Moscow.
The separatists initially described the monitors, led by a German colonel, as prisoners of war. One Swede was freed earlier on health grounds while four Germans, a Czech, a Dane and a Pole were still being held until Saturday. A Russian envoy helped negotiate their release.
The separatist leader in Slaviansk, self-proclaimed "people's mayor" Vyacheslav Ponomaryov, said they were freed along with five Ukrainian captives, with no conditions. "As I promised them, we celebrated my birthday yesterday and they left. As I said, they were my guests."
Western countries blame Russia for stoking the separatism and fear Moscow could be planning to repeat its annexation of Crimea in other parts of Ukraine. Russia denies it has such plans, while saying it could intervene if necessary to protect Russian speakers, a new doctrine unveiled by President Vladimir Putin in March that overturned decades of post-Soviet diplomacy.