Ukrainian prime minister Arseniy Yatseniuk says Russia is seeking to destroy Ukraine by engineering clashes, including the fire that killed 42 mostly pro-Russian activists in Odessa.
Yatseniuk visited Odessa on Sunday, two days after the deadly blaze in the southern port city of one million people. He accused Moscow of engaging in "well-planned provocation" against the interim Kyiv government.
"Destruction of the country and splitting the country - this is the Russian plan. Odessa was holding on for two months until they (the Russians) sent more people here to heat up the situation. Odessa is far too valuable for Russia. But it is even more valuable for us because it is our Odessa, our country and our people.''
Ukraine says it will continue pressing its military offensive against pro-Russian separatists in eastern Ukraine, as the Kremlin reported receiving thousands of calls for help from the region's Russian-speaking citizenry.
Ukraine Interior Minister Arsen Avakov said troops had recaptured a television tower and government buildings from rebels in Kramatorsk, a town near the pro-Russian stronghold city of Slovyansk.
Ukraine's National Security and Defense Council chief Andrily Parubiy said an anti-terrorist operation will be carried out in towns beyond Slovyansk and Kramatorsk.
In Moscow, a Kremlin spokesman (Dmitri Peskov) said Russian President Vladimir Putin had not yet decided how to respond to the offensive, or to the deaths in Odessa.
Russia currently has at least 40,000 troops and armor massed on its border with Ukraine, and the Kremlin says it reserves the right to enter the country to protect ethnic Russians.
A German newspaper, Bild am Sonntag, citing unnamed German security sources, said the U.S. Central Intelligence Agency and the Federal Bureau of Investigation are providing dozens of specialists to the Ukraine government to set up a security structure in the country and to fight organized crime.
In Kyiv, the interim government declared two days of mourning for those killed Friday in fighting in Odessa.
Witnesses say hours of street battles in the Black Sea port city ended in a deadly blaze at a trade union building where overmatched separatists had holed up seeking safety. It remains unclear who started the fire, but witnesses said gasoline bombs exploded around the building during the melee and gunfire was heard.
On Saturday, U.S. Secretary of State John Kerry praised the release of European monitors abducted late last month by separatists near Slovyansk.
Kerry said Russia must now withdraw support for the separatists. Moscow says Kerry's Russian counterpart, Sergei Lavrov, countered that Washington should compel the Kyiv government to stop its military offensive in the east.
U.N. Secretary-General Ban Ki-moon also welcomed the freeing of the seven observers with the Organization for Security and Cooperation in Europe. The monitors were freed Saturday, along with their five Ukrainian assistants.
And in Brussels, the European Union called for an independent investigation into the Odessa deaths. EU foreign policy chief Catherine Ashton said member-countries were "deeply saddened" by the deaths and injuries.