Ukraine's acting President Oleksandr Turchynov has demanded that Russia stop meddling in Ukraine's affairs and halt its "blackmail."
In a televised statement on April 24, the president demanded that Moscow pull back its troops from the Ukrainian border.
He accused Russia of "coordinating and openly supporting terrorist killers" in eastern Ukraine, and said Ukraine will not "back down from the terrorist threat."
Earlier on April 24, Russia announced the start of new military drills near the Ukrainian border after Ukraine's government launched an offensive against pro-Russian separatists in eastern Ukraine.
Defense Minister Sergei Shoigu said the Russian drills were in response to the Ukrainian offensive, as well as NATO exercises in Poland and the Baltic states.
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Russian President Vladimir Putin said the Ukrainian offensive constitutes a "serious crime" and would entail consequences.
Russia denies Ukrainian and Western allegations that it has sent agents into Ukraine to stir up separatist unrest. But Putin has said Russia has the "right" to intervene in Ukraine to protect ethnic Russians there.
Western officials have said Russia has deployed as many as 40,000 troops near the Ukrainian border since pro-Russian Ukrainian President Viktor Yanukovych was ousted in late February.
Earlier on April 24, U.S. President Barack Obama accused Moscow of failing to live up to "the spirit or the letter" of an international agreement signed last week.
The deal was signed April 17 by Ukraine, Russia, the United States, and the European Union.
NATO last week announced it was boosting its ground, sea, and air forces in eastern members of the alliance to respond to what Western officials describe as Russia's interference in Ukraine.
The United States, which has already sent fighter jets to Poland, has announced it is sending 600 troops to Poland and the Baltic states to participate in military exercises. On April 23, 150 U.S. paratroopers arrived in Poland.
UN Secretary-General Ban Ki-moon, in a statement April 24, warned the situation in eastern Ukraine "could quickly spin out of control."
Ukraine's Interior Ministry says five separatists were killed in clashes near the flashpoint city of Slovyansk early on April 24. The separatists have confirmed the deaths of two of their men.
Ukrainian media reports say security forces have now withdrawn from checkpoints near Slovyansk they had seized earlier. The reason for the withdrawal was not immediately clear.
With reporting by Reuters, AP, and AFP