Ukraine declares two days of mourning after bloodshed

Ukraine's interim president on Saturday declared two days of mourning after more than 50 people died in the country's bloodiest day in months.

"The day of May 2 was a tragic day for Ukraine. I have signed a decree for two days of mourning in Ukraine for the heroes who died in the course of the anti-terrorist operation and also for those who died in the tragic events in Odessa," Oleksandr Turchynov said in a statement.

At least nine people, including four servicemen, died in fighting on Friday as the Ukrainian army intensified what Kiev calls an "anti-terrorist" operation around the rebel-held town of Slavyansk.

And at least 42 people lost their lives in the southern port city of Odessa after fighting between pro-Russian and pro-Kiev militants climaxed in an inferno that trapped dozens as both side hurled petrol bombs.

The Kremlin and Ukraine traded accusations over the violence, with Kiev saying the Odessa violence had been "coordinated by sabotage groups from Russia".

Moscow said it was "outraged" as the scenic port city became a new front in an escalating months-long crisis that has sparked fears of a Russian invasion.

The foreign ministry in Moscow called on Ukraine and its "Western backers to end the anarchy and take responsibility before the Ukrainian people", blaming Kiev's "criminal irresponsibility" for the sinister turn of events.