Ukraine's security service said in Kiev on Saturday illegal military groups from Moldova's breakaway region of Transdniestria and Russian groups worked together to foment unrest in the southern port city of Odessa.
"The unrest, which occurred on May 2 in Odessa and led to clashes and many casualties, was due to foreign interference," a spokeswoman for the SBU security service told a news conference.
She said former top officials, once part of ousted Ukrainian President Viktor Yanukovich's inner circle, had financed "saboteurs" to foment the unrest, blaming Serhiy Arbuzov and Oleksander Klymenko, who were now "hiding in a neighboring country".
On the ongoing Ukrainian military operation in the east the deputy chief of the SBU, Vasyl Krutov said the army were in control of parts of Slaviansk where separatists had been holding OSCE representatives who were released on Saturday.
But he said the operation was very complex.
"Today we are in total control of the internal radius of this city (Slaviansk) where 14 checkpoints have been set up, controlled by the forces of the anti-terrorist operation," Krutov said.
Following reports that rebels took over the eastern town of Kramatorsk, Ukrainian forces announced they had retaken the security service headquarters, the country's Interior Ministry said on Saturday.
"Now it is under the control of the National Guard," the ministry said in a statement. Local television showed pictures of armored personnel carriers moving through the town.
The United States publicly condemned the violent death of more than 40 Ukrainians on Friday night, saying that the country "mourns with all Ukrainians the heartbreaking loss of life in Odessa."
"Today the international community must stand together in support of the Ukrainian people as they cope with this tragedy. The violence and mayhem that led to so many senseless deaths and injuries is unacceptable," a statement read.
The State Department stressed that the events in Odessa which led to a fire in the Trade Union Building proved the need for an immediate de-escalation of tensions in Ukraine and once again called for the immediate implementation of commitments made in Geneva on April 17.
Ukrainian troops launched a dawn raid to try to retake the rebel stronghold of Slovyansk on Friday, surrounding the eastern town and sending in military helicopters, one of which was shot down by pro-Russian rebels, killing two servicemen.
Accusing the pro-Moscow separatists of using mercenaries, Interior Minister Arsen Avakov said the offensive was answered with heavy artillery, with the rebels using grenade and portable anti-aircraft missile launchers to bring down the plane.