At least 38 people -- most of them believed to be pro-Russian activists -- died in a fire in a trade union building in the southern port city Odesa on May 2, RFE/RL reports.
It is unclear how the blaze began, but some reports say Molotov cocktails had been thrown at the building. Television footage showed petrol bombs exploding against the walls of the building.
Earlier, there had been clashes between pro-Russian separatists and supporters of the Kyiv authorities in areas of Odesa. An Interior Ministry statement said 42 people in total had been killed on May 2, but did not give a breakdown.
Acting Ukrainian President Oleksandr Turchynov has declared two days of mourning for those killed.
On May 3, Ukraine's State Security Service (SBU) blamed "foreign interference" for the clashes in Odesa. An SBU spokeswoman alleged that illegal military groups from Moldova's breakaway region of Transdniester and Russian groups had worked together to foment the unrest.