Ukrainian Prime Minister Arseny Yatsenyuk has accused Moscow of seeking to start World War III with its "aggressive" actions in Ukraine.
Speaking at a cabinet meeting on April 25, he said Russia aims to occupy Ukraine "militarily and politically" and spark a war there that "will lead to a military conflict in Europe."
He also said Moscow's support for pro-Russian separatists in Ukraine's east "constitutes an international crime" and urged the international community to "unite against Russian aggression."
Yatsenyuk's comments came as acting Defense Minister Mykhailo Koval said Russian troop maneuvers have come within one kilometer of Ukraine's border.
Koval added that Ukrainian armed forces stand "ready to repel any aggression."
Western nations have slammed Russia for massing an estimated 40,000 troops along the Ukrainian border.
Russia claims it is conducting military exercises in response to the "antiterrorist" operation that Kyiv has launched to dislodge pro-Russian insurgents occupying government buildings in a dozen eastern Ukrainian towns.
Serhiy Pashynsky, a top aide to acting President Oleksandr Turchynov, is quoted by the news agency Interfax-Ukraine as saying that any incursion by Russian forces across the border will be considered an invasion and the attackers will be killed.
"We do not accept false declarations about humanitarian action," he said.
War Of Words
Russian Foreign Minister Sergei Lavrov, who has been waging a war of words with the West, accused Washington of distorting an international agreement on Ukraine by making additional demands on Moscow.
He said Russia remains committed to helping de-escalate the conflict "based on the compromise approach agreed in Geneva."
The agreement calls for all illegal armed groups in Ukraine to be disbanded and all occupied buildings to be vacated.
But Lavrov charged that "one-sided demands" are being made by the United States, which he said has "an outstanding ability to turn everything on its head."
He also said the pro-Western government in Kyiv will eventually face justice for its security operation in eastern Ukraine, describing it as a "bloody crime."
Speaking on April 25 in South Korea, U.S. President Barack Obama said he will consult with key European leaders later in the day to discuss the Ukraine crisis and additional possible sanctions against Russia.
A German government spokesman says Chancellor Angela Merkel believes Russia's actions regarding Ukraine since last week's Geneva agreement have been "absolutely disappointing."
The spokesman said Merkel conveyed her "great concern" to Russian President Vladimir Putin in a phone call on April 25.
The Geneva statement, intended to de-escalate tensions in Ukraine, was agreed on April 17 by the foreign ministers of Russia, Ukraine, the European Union, and the United States, but few measures have been implemented so far.
On April 24, U.S. Secretary of State John Kerry warned that the "window of opportunity" for Russia to change course in Ukraine is closing.
Kerry accused the Kremlin of seeking to undermine Ukraine's democratic processes and said the consequences could be "grave" and "expensive" for Russia.
Meanwhile, on the ground in Ukraine, officials in Kyiv say one of their military helicopters was hit by gunfire and exploded on the tarmac at a base near the eastern town of Kramatorsk on April 25. The pilot managed to escape but was wounded.
Kramatorsk is one of several towns under the control of pro-Kremlin gunmen.
With reporting by Reuters, AP, AFP, Interfax, and ITAR-TASS