Ukraine’s interim President Oleksandr Turchynov ordered the resumption of military operations in eastern Ukraine Tuesday after two people, one of them a politician, were allegedly kidnapped, tortured and killed by pro-Russian separatists and later found inside a military aircraft reportedly hit by gunfire.

The bodies were found in Slovyansk and one of the victims was identified as Vladimir Rybak, a politician belonging to Turchynov’s party and a member of parliament, who had gone missing recently. The incident occurred just hours after U.S. Vice President Joe Biden had left Ukraine after meeting with Turchynov and Prime Minister Arseniy Yatsenyuk.

"I call on the security bodies to resume and carry out successful anti-terrorist measures aimed at defending Ukrainian citizens living in the east of Ukraine against terrorists," Turchynov reportedly said in a statement, according to BBC.

He went on to add that terrorists "are beginning to torture and kill Ukrainian patriots. They are impudently rejecting the calls of not only our country but of all the world's society when they demonstratively mock the decisions taken in Geneva," Associated Press, or AP, reported. "These crimes are being done with the full support and connivance of Russia."

Meanwhile, U.S. Secretary of State John Kerry threatened more sanctions against Russia if it did not de-escalate tensions in Ukraine. Kerry spoke to Russian Foreign Minister Sergei Lavrov over the phone Tuesday and expressed "deep concern over the lack of positive Russian steps." Kerry also said that “the absence of measurable progress on implementing the Geneva agreement will result in increased sanctions on Russia," BBC reported, citing a state department official.

Lavrov has denied the claims and reiterated that Russia was not behind the violence in eastern Ukraine.

"Before putting forth ultimatums to us, demanding fulfillment of something within two-three days or otherwise be threatened with sanctions, we would urgently call on our American partners to fully recognize responsibility for those whom they brought to power and whom they are trying to shield, closing their eyes to the outrages created by this regime and by the fighters on whom this regime leans," he said, according to AP.

During Biden's two-day visit, the vice president announced that the U.S. will provide an additional $50 million, including $11 million to fund presidential elections slated for May 25, to the Ukrainian government. Biden also announced $8 million in nonlethal military assistance such as bomb-disposal equipment, communications gear and vehicles for the country’s armed forces.

“We’ve heard a lot from Russian officials in the past few days, but now it’s time for Russia to stop talking and start acting. Act on the commitments that they made:  to get pro-Russian separatists to vacate buildings and checkpoints, accept amnesty and address their grievances politically; to get out on the record calling for the release of all illegally occupied buildings,” Biden said Tuesday in Ukraine, according to a statement from the White House.