Ukraine's acting president has ordered a military operation against pro-Kremlin separatists in the east of the country, after the body of a local politician from his own party was found showing signs of torture.
"I demand the resumption of effective counter-terrorism measures to protect Ukrainian citizens in the east from terrorism," Oleksandr Turchynov said on Tuesday, according to a statement from his other office as parliamentary speaker.
Turchynov said that "brutally tortured" bodies had been found near the city of Slaviansk, which is in the hands of pro-Russian militants.
|Al Jazeera's Jacky Rowland reports from Donetsk
One was that of Volodymyr Rybak, a member of Turchynov's Batkivshchyna party, who had recently been abducted by "terrorists".
"These crimes are being carried out with the full support and indulgence of the Russian Federation," he said.
Turchinov's comments came as the US deployed troops to Poland to begin a series of military exercises in four countries across eastern Europe.
Rear Admiral John Kirby, a Pentagon spokesman, said the exercises would last about a month, and involve about 600 troops.
About 150 soldiers from the 173rd Airborne Brigade based in Vicenza, Italy, will start the exercises on Wednesday in Poland. Similar exercises will be conducted in Estonia, Latvia and Lithuania and troops could arrive in those countries by Monday.
The exercises are part of an effort announced last week aimed at reassuring NATO allies of the US's commitment to the region's defence.
Call for pullout
Earlier on Tuesday, Arseny Yatsenyuk, the interim Ukrainian prime minister, said Russian special forces were operating in eastern Ukraine to undermine a presidential election due on May 25, and called on Moscow to pull them out.
At a news conference after meeting Joe Biden, the US vice president, Yatsenyuk also urged Russia to remove its troops from Crimea, annexed by Moscow last month.
"Everything that is now happening in the east and which Russia is supporting is aimed at wrecking the presidential election," Yatseniuk said, as he called on Russia to fulfil its obligations.
"We don't ask anything from Russia. We demand only one thing: fulfil international agreements and don't behave like gangsters."
Biden said that Ukraine must remain "one united Ukraine", and criticised Russia's takeover of Crimea.
"No nation has the right to simply grab land from another nation," he said. "We will never recognise Russia's illegal occupation of Crimea, and neither will the world."
Biden also said Russia needed to defuse confrontations in the east of Ukraine.
"No nation should threaten its neighbour by massing troops on the border. We call on Russia to pull these forces back. We call on Russia to stop supporting men, hiding behind masks, sewing unrest in eastern Ukraine."
Biden's visit comes at a critical time, just days after a tenuous international agreement was reached to de-escalate violence in eastern Ukraine, where pro-Russia rebels oppose the government in Kiev.
Under the international deal, signed by Ukraine, Russia, the US and the EU, rebels were to disarm and give up buildings they seized in the east of Ukraine.
In another development, a Ukrainian military surveillance plane was struck by gunfire over the contested eastern city of Slovyansk, but landed safely with no one injured, the country's Defence Ministry said.
The Antonov AN-30 was hit multiple times by shots from armed pro-Russian forces, the ministry added.