Ukrainian officials have alleged that the mass killing of protesters in Kyiv in February took place on the orders of ousted president Viktor Yanukovych, and that Russian security service agents helped him plan and carry out the assault.
Acting Interior Minister Arsen Avakov told reporters in Kyiv Thursday that Mr. Yanukovych "issued the criminal order ... to open fire against protesters on February 18-20."
Speaking at the same news conference, the head of Ukraine's security service (Valentyn Nalyvaychenko) said there were grounds to believe that agents of Russia's Federal Security Service, or FSB, were involved in "the planning and implementation of the so-called anti-terrorist operation" in the Ukrainian capital in late February.
Ukraine's government also announced Thursday the detention of 12 members of an elite riot police unit on suspicion of shooting protesters.
More than 100 people were killed during February's unrest in Kyiv, many of them apparently by snipers.
The FSB responded to the Ukrainian charges Thursday, with an unnamed spokesman telling Russia's state news agency RIA Novosti: "Let those allegations remain on the conscience of the Ukrainian Security Service."
Mr. Yanukovych, who fled Kyiv in February after months of anti-government protests, said Wednesday he was "wrong" to invite Russian troops into Ukraine's Crimean peninsula.
In his first interview since he sought refuge in Russia, Mr. Yanukovych told the Associated Press and Russia's NTV television that he will try to persuade Moscow to return the territory to Ukrainian control.
Meanwhile, Russian Foreign Minister Sergei Lavrov said Thursday that Russian troops deployed near the Ukrainian border will return to their permanent bases as soon as they finish military exercises.
NATO's top commander, General Philip Breedlove, said Wednesday that the 40,000 Russian troops in the border are area "ready to go" and could take over large parts of Ukraine in as little as three days.
Ukrainian officials have said as many as 100,000 Russian soldiers along its eastern and southern borders are poised to invade.
Lavrov Thursday called on the West and Ukraine to de-escalate the rhetoric, which he says is becoming unreasonable.
He also asked NATO for answers on its plans to beef up defenses in Eastern Europe -- a direct response to the Russian takeover of Crimea.
Meanwhile, Gazprom, Russia's state-owned natural gas export monopoly, said Thursday that it was hiking the price of natural gas shipments to Ukraine another 26 percent, just two days after announcing a 44-percent price hike.
Ukrainian Energy Minister Yuri Prodan said the hikes were excessive and amounted to a "political price."