* Ukraine president calls for new crackdown in east
* Follows discovery of party ally's tortured body
* Past operation was limited but could hamper peace efforts
* International mediators make slow progress after Geneva (Adds Interior Ministry statement)
By Aleksandar Vasovic and Alissa de Carbonnel
SLAVIANSK/DONETSK, Ukraine, April 22 (Reuters) - Ukraine's acting president accused Russia of aiding "terrorists" who killed a politician from his own party, urging a crackdown in the east on Tuesday that could hamper international efforts to defuse the crisis.
As Vice President Joe Biden flew home from a brief visit to demonstrate solidarity with Kiev's new leaders against Moscow, Oleksander Turchinov called on security forces to relaunch an operation against pro-Russian separatists that was suspended after a deal with Moscow last week on disarming militants.
In fact, Ukraine's poorly resourced forces had previously shown little sign of taking on the gunmen who started occupying towns and public buildings two weeks ago. Turchinov's call may not lead to much more action. But it may fuel recriminations between Moscow and Kiev about who is failing to honour the deal.
Describing marks of "brutal torture" on the body of kidnapped Batkivshchyna party politician Volodymyr Rybak when found near the separatist stronghold of Slaviansk, Turchinov said in a statement: "These crimes are being carried out with the full support and indulgence of the Russian Federation.
"I call on the security agencies to relaunch and carry out effective anti-terrorist measures, with the aim of protecting Ukrainian citizens living in eastern Ukraine from terrorists."
Since Russia signed up to the four-way accord in Geneva along with Ukraine and Kiev's U.S. and EU allies, militants who deny taking orders from Moscow and whom Russia denies it controls, have if anything tightened their grip on about a dozen buildings and towns, mostly in the industrial Donetsk region.
Mediators from Europe's security body, the OSCE, have been charged with negotiating an end to the standoff in Russian-speaking eastern Ukraine, which follows the overthrow of the Kremlin-backed president in Kiev two months ago and Russia's subsequent occupation and annexation of the Crimea peninsula.
Having met the leader of separatists in Slaviansk on Monday, the chief OSCE negotiator met those occupying the government headquarters in the regional capital Donetsk. One of them told reporters they had agreed to vacate the council chamber and two floors of the 11-storey block, though details were unclear.
In Kramatorsk, near Slaviansk, the Ukrainian Interior Ministry condemned the abduction of the local police chief and the takeover of the SBU security service building the previous day. It said Kramatorsk's police chief was being "held hostage".
Militants in Slaviansk buried three men killed in a gunfight in the early hours of Sunday that Kiev and Moscow accused each other of staging deliberately to hinder peace negotiations.
Hundreds of people turned out at a local church. Men in blue uniforms and Cossack hats carried the coffins. Several masked gunmen in camouflage were also in attendance. The Ukrainian government says Russian special forces are in the city.
There were tears and chants of "Rossiya!" - Russia - from people who want the Donbass coal and steel belt around Donetsk and Luhansk regions to follow Crimea into union with Moscow.
"They gave their lives for greater rights for all of us in the Donbass," said Dmitry, 26, who wore a camouflage coverall and a vest filled with ammunition clips in pockets. "They died for our freedom and our future in great Russia."
"Rest assured, we'll have our revenge," another gunman said.
One mourner shouted at journalists: "Look at these bodies. Report the truth. I am warning you - tell the truth."
Separatist militants have detained several journalists for short periods and are holding at least two for longer. The self-declared mayor of the Slaviansk, Vyacheslav Ponomaryov, told a news conference on Tuesday that an American had been detained.
Vice News, an online news site, said it was in touch with the U.S. State Department to secure the safety of Simon Ostrovsky, who has been covering the Ukraine crisis for weeks and whom fellow journalists saw being detained on Monday.
Slaviansk has been a focus of violence. The Ukrainian Defence Ministry said one of its aircraft was hit by bullets on Tuesday during a reconnaissance flight over the city.
The Interior Ministry said Rybak's was identified as one of two bodies found in a river on Saturday. Both appeared to have drowned after being tortured. It said Rybak had an altercation with separatists after a demonstration on Thursday in favour of Ukrainian unity in Horlivka, where he was a town councillor.
He was last seen later that day being forced into a car by four masked men in camouflage. The ministry concluded that both men were tortured and killed by militants from Slaviansk.
Acting president Turchinov said: "Terrorists who have effectively taken the whole Donetsk region hostage, have crossed a line, starting to torture and murder Ukrainian patriots.
"They throw down an insolent challenge not only to our country but to the entire international community when they commit blatant outrages against the decisions taken in Geneva."
Like Turchinov, Rybak was a member of the Batkivshchyna, or Fatherland, party led by Yulia Tymoshenko. A former prime minister, she was jailed for corruption under President Viktor Yanukovich and freed when the Moscow-backed leader fled from protests triggered by his rejection of closer ties with the EU.
Tymoshenko herself, running for president in an election due on May 25, was in Donetsk on Tuesday. Her aides tried to set up a news conference close to the occupied regional government building but were forced to change venue after a hostile crowd sprayed a fire hose on the area and pelted them with eggs.
Tymoshenko lost the 2010 presidential election to Yanukovich, whose power base is in his home town of Donetsk. She is seen by many Russian-speakers in the east as an advocate for Ukrainian-speaking nationalists in the west of the country. (Additional reporting by Natalia Zinets and Alastair Macdonald in Kiev and Lina Kushch in Donetsk; Writing by Alastair Macdonald; Editing by Will Waterman)