Ukraine is stepping up security as the country gets set to celebrate Victory Day on May 9.
Authorities have discouraged public gatherings, fearing they could spark clashes between pro-Russian activists and supporters of Ukrainian unity.
Ukrainian Prime Minister Arseny Yatsenyuk said on May 8 he suspects Russian President Vladimir Putin was planning some form of "skirmish" to discredit Kyiv as the country celebrates the 69th anniversary of the defeat of Nazi Germany in 1945.
Yatsenyuk spoke after pro-Russian separatists in Donetsk and Luhansk in eastern Ukraine said they will go ahead with referendums on self-determination on May 11 despite calls by Putin to postpone them.
Putin's spokesman said the Kremlin needed more information about the rebels' decision.
A spokeswoman for EU foreign-policy chief Catherine Ashton, said the referendums "should not take place neither on May 11 or any other later date."
U.S. Deputy Secretary of State William Burns said Russia was heading down a "dangerous and irresponsible path" and the situation in Ukraine was "extremely combustible."
Burns spoke after NATO again said it had seen no Russian withdrawal of troops along the border with Ukraine as Putin has vowed to do.
Elsewhere, some 40 armed men attacked a Ukrainian post on the Russian frontier on May 8.
The Ukrainian border guard said the attackers were beaten off by Ukrainian forces.
Meanwhile, Ukrainian leaders have proposed the creation of a "Round Table" to resolve the crisis in the country.
The announcement was made on May 8 by Yatsenyuk and acting President Oleksandr Turchynov.
The round table, the two leaders said, would draw in political forces and civil groups in all regions, with international mediators asked to help.
Analysts say the Ukrainian plan includes many elements of a draft peace "Road Map" presented on May 8 in Kyiv by the Swiss chair of the Organization for Security and Cooperation in Europe.
Efforts to defuse the Ukraine crisis were also discussed by Russian Foreign Minister Sergei Lavrov and U.S. Secretary of State John Kerry in a telephone conversation on May 8.
Based on reporting by Reuters and dpa