Ukrainian forces cleared armed Pro-Russian activists from a public building in Donetsk late Monday.
This following a speech by the country's acting President Oleksanr Turchynov in which he vowed to repel efforts to "dismember" his country.
Pro-Russian activists in the eastern city of Donetsk had been proclaiming the creation of a separatist Donetsk republic.
One of the activist leaders called for a referendum to be held by May 11th at the latest.
"Aiming to create a people's legal, democratic, sovereign state we proclaim a creation of a sovereign state-the People's Republic of Donetsk."
Others said they will call on Russia to send in a peacekeeping force if Kiev authorities respond to their demands with aggression.
"We are addressing you, Vladimir Vladimirovich as our last hope for our future and for the future of our children."
Two other eastern cities -- Kharkiv and Luhansk -- were also taken over by pro-Russian activists, who made similar demands for referendums.
Ukrainian Prime Minister Arseny Yatseniuk called the action part of a Russian-written script to continue to seize parts of Ukraine,.. following its takeover of Crimea.
The U.S. is warning Russia against moving into eastern Ukraine and accused it of backing the separatist movement in Ukraine.
"There is strong evidence suggesting some of these demonstrators were paid and were not local residents We call on President Putin and his government to cease all efforts to destabilize Ukraine."
U.S. Secretary of State John Kerry also told Russian Foreign Minister Sergei Lavrov over the phone, that Washington was deeply concerned by the recent events.
He added that further moves by Moscow to destabilize the region would incur more costs for Russia.
Yoo Li-an, Arirang News.