Update as of 6:34 a.m. EDT: Five pro-Russian rebels were killed in a clash at a checkpoint in the eastern Ukraine city of Slovyansk, the Wall Street Journal reported, citing Ukraine's interior ministry, and added that Russian President Vladimir Putin had called Ukraine's use of military force a "serious crime." The Journal, citing Interfax, also reported a Russian politician as saying that Russia will not send troops into eastern Ukraine.

U.S. President Barack Obama said Thursday that the U.S. and its allies have additional sanctions ready to be deployed against Russia, even as the first set of 150 U.S. troops, who would be later joined by 450 more, reached Poland on Wednesday for military exercises.

The military exercises would last a month, the New York Times reported Wednesday, citing the Pentagon press secretary, even as violence in eastern Ukraine continued despite the signing of a pact last Thursday in Geneva. Ukraine's decision to respond to the alleged actions of pro-Russian protesters -- including the killing of two people in Slovyansk and the kidnapping of nearly seven journalists -- with military force has again raised the prospect of a retaliatory move by Russian troops, even as the U.S. and European Union, which blame Russia for the unrest, readied further sanctions against Russia.

"It seems Russia's intentions are quite clear. What we're seeing is modern warfare that may include civilians, some special forces, intensive propaganda, and may be as effective as tanks," Polish Prime Minister Donald Tusk said Wednesday, according to the Wall Street Journal.

Obama, who is on a four-nation tour in Asia, said at a conference in Tokyo, according to Associated Press: "So far we have seen them not abide by the spirit or the letter of the agreement in Geneva,” adding that if Russia continued to violate the pact, "there will be further consequences and we will ramp up further sanctions."

Obama also said, according to Bloomberg, that, “There’s always the possibility that Russia tomorrow or the next day takes a different course,” adding: “We have been preparing for the prospect that we’re going to have to engage in further sanctions.”

Obama’s comments followed a statement from Russian Foreign Minister Sergei Lavrov, who blamed the unrest on Ukraine’s government and said that it was the U.S., which was “running the show” in Ukraine, and stated that his government would respond if it were attacked.

“Nothing which was agreed in Geneva and which certainly is for the authorities in Kiev to start implementing was done by them,” Lavrov said Wednesday, in an interview with Russia Today, a local news network. "It’s quite telling that they chose a moment of vice-president of the United States visit to announce the resumption of this operation because the launching of this operation happened immediately after the John Brennan’s visit to Kiev. So I don’t have any reasons not to believe that the Americans are running the show in a very close way.”

Meanwhile, the interim administration in Kiev appealed again to Russia to recall its troops, while Ukraine's foreign ministry provided an explanation for its decision of placing troops in parts of eastern Ukraine.

“There is a threat that the armed groups of the pro-Russian forces in this part of Ukraine could take advantage of the Russian troops' presence in the vicinity of the border in order to provoke their sending into the territory of Ukraine,” the country’s foreign ministry said Wednesday, according to Interfax, a local news agency. “The presence of Ukrainian Armed Forces units in the eastern regions of our state is exclusively aimed at protecting our citizens irrespective of their nationality. Furthermore, the decision to put them on high alert comes as a logical response to the threats of force use and [possible] aggression against the territorial integrity of our state that have been heard recently from official representatives of the Russian Federation.”