Russian forces have tightened their grip on Crimea, as authorities in the breakaway territory push their plan to join Moscow.
Ukraine's interim prime minister vowed Sunday not to give up "a single centimeter" of territory. Arseniy Yatsenyuk addressed supporters who rallied in Kyiv to celebrate the 200th birthday of Ukrainian poet and national hero Taras Shevchenko.
Mr. Yatsenyuk is to meet Wednesday with U.S. President Barack Obama in Washington to discuss the standoff over Crimea, a strategic peninsula in southern Ukraine with a Russian-speaking majority.
A Russian lawmaker said the Kremlin had set aside $1.1 billion to rebuild Crimea's industrial infrastructure if the disputed region votes in a March 16 referendum to join Russia.
German Chancellor Angela Merkel told Russian President Vladimir Putin in a phone call Sunday the planned Moscow-backed referendum is illegal and violates Ukraine's constitution.
Mr. Putin defended the separatist drive in Crimea as consistent with international law, and a regional leader said Ukrainian troops remaining there should leave the territory unless they renounce their loyalty to Kyiv.
U.S. Deputy National Security Adviser Tony Blinken says Washington will not recognize the annexation of Crimea by Russia if residents of the region vote to leave Ukraine.
Former U.S. ambassador to Russia Michael McFaul told VOA the annexation will "isolate Russia from the rest of the world for years to come, maybe even decades to come." He said "Even the Chinese are not supporting Russia in this act - nobody thinks this act is legitimate."
Russian forces tightened their grip on the peninsula taking over a Ukrainian border post on the western edge of Crimea, trapping about 30 personnel inside. A Ukrainian military spokesman said Russian forces now control 11 border guard posts across the territory.
As demonstrators staged rival rallies in Crimea and throughout Ukraine, street violence flared in Sevastopol when pro-Russian activists and Cossacks attacked a group of Ukrainians.
In the ethnic Russian eastern Ukrainian city of Donetsk, the Kyiv Post says 7,000 demonstrators in Lenin Square tore down a Ukrainian flag, replacing it with the Russian tricolor ensign and chanting "Putin our president" and "referendum."
In Kyiv, former imprisoned Russian tycoon Mikhail Khodorkovsky implored a pro-Western crowd to believe that not all Russians support Moscow's recent actions in Ukraine.
Russia denies it has troops on the peninsula beyond those regularly stationed with its Sevastopol-based Black Sea fleet. Ukraine's much smaller navy is also based in the Crimean port city.
Witnesses say although the soldiers have no insignia identifying them, they are clearly Russian.
Foreign observers have failed to get into Crimea to get a first-hand look at the situation and were forced to turn back Saturday after pro-Kremlin gunmen fired warning shots.