Ukraine has formally called off the Easter truce against pro-Russian separatists in the east and says anti-terrorist operations are on again, while Russia threatens to retaliate.
First Deputy Prime Minister Vitaliy Yarema told reporters during a Cabinet meeting Wednesday that what he calls "appropriate steps" will be taken and that results will be seen shortly.
Yarema said Ukraine had received assurances from the United States that it would not be left alone to face Russian aggression.
On Tuesday, Ukraine's acting President Oleksandr Turchynov ordered a resumption of "anti-terrorist" operations against pro-Russian separatists who continue to occupy government buildings in several eastern Ukrainian cities.
The presidential order followed the discovery of two bodies near the rebel-held city of Slovyansk. One of the victims was identified as a local member of Mr. Turchynov's political party who was reportedly abducted last week.
Mr. Turchynov said the victims had been "brutally tortured" and that such crimes are being committed "with the full support and connivance of the Russian Federation."
In Moscow, Russian Foreign Minister Sergei Lavrov told state television that Russia would retaliate if its legitimate interests or the Russian people were attacked. He said Russia's response would be the same as it was in South Ossetia in 2008, which led to a brief war with Georgia.
In a separate statement Wednesday, Russia's Foreign Ministry said Kyiv and Washington had "distorted interpretations" of an agreement signed in Geneva last week that aimed to reduce tensions in Ukraine.
The ministry said Ukraine, the United States and European countries were insisting that only pro-Russian forces disarm while "turning a blind eye to the continuing provocative actions by militants of right-wing forces" -- in particular, those belonging to the ultra-nationalist group Right Sector.
The Pentagon said Tuesday it was sending 600 U.S. troops to Poland and the Baltics to "send a message" to Moscow and to reassure American allies and partners.
U.S. Vice President Joe Biden ended a two-day visit to Kyiv Tuesday, calling on Moscow to honor its promise to withdraw support for armed pro-Russian separatists in Ukraine.
Ukraine's military failed last week to drive the separatists from government buildings they have seized in about a dozen cities.
Pro-Russian gunmen are demanding the right to hold referendums on splitting with Ukraine and joining Russia. A vote last month in Crimea led to the Russian annexation of the Ukrainian peninsula.