Activists chanting "Russia!" broke through police lines Sunday and stormed several government buildings in eastern Ukrainian regions seeking independence from Kiev following last month's fall of a Kremlin regime.

Clashes in Donetsk and similar rallies in the heavily Russified cities such of Lugansk and Kharkiv provided another reminder to the untested pro-Western leaders in Kiev of the monumental task facing them after their February 22 overthrow of president Viktor Yanukovych.

The body of a far-right Ukrainian activist and reporter was found dumped in the woods with signs of torture a day after his abduction, his party said on Sunday.

The blood-caked and bruised remains of Svoboda (Freedom) ultra-nationalist party member Vasyl Sergiyenko were found under a pile of rubbish in the woods near the central Ukrainian village of Vygrayev, 120 kilometers (75 miles) southeast of Kiev, the party said in a statement.

Ukraine's security service said on Saturday it had arrested 15 men for allegedly plotting to stage an armed revolt in an eastern region neighboring Russia.

It added it had also seized 300 machineguns, a rocket launcher and numerous grenades from the suspected coup plotter.

Russia needs to understand it is not in its interest "to have a collapsing state in its neighborhood", German Foreign Minister Frank-Walter Steinmeier said at the close of two-day talks with his 27 European Union counterparts.

As the Ukraine-Russia dispute over gas prices intensified, Germany's Steinmeier stressed that Europe needed Russia to help rescue the new authorities in economically distressed Kiev.

Russia's top intelligence agency admitted Saturday that a high-ranking officer was in Kiev during February bloodshed, but said he was there to ensure "security" of the Russian embassy.

"We confirm that (FSB general) Sergei Beseda was in Kiev on February 20-21... to determine the level of security necessary for the Russian embassy in Ukraine and other Russian facilities in Kiev," Russian agencies quoted a source in the powerful Federal Security Service (FSB) agency as saying.

Rating firm Moody's again lowered Ukraine's credit rating by a notch on Friday, citing the "escalation" of its political crisis, and put the country on a "negative" outlook for further downgrades.

Moody's Investors Service pushed the country's rating deeper into speculative territory, to "Caa3" from "Caa2" -- a one-notch move matching the rating firm's prior downgrade in January.

"Europe must not relax" on the Ukraine crisis as the situation remains tense with Russian troops making only "a token withdrawal" from the border, British Foreign Secretary William Hague said Friday.

"Any withdrawal of the forces has been only a token withdrawal," said Hague on arriving for talks with his 27 European Union counterparts.

Crimea on Thursday said it was opposed to an autonomous territory for the Tatars, an ethnic minority that was against the Black Sea peninsula's recent annexation by Russia.

"No, that is not possible, there can only be a cultural autonomy," Crimean deputy prime minister Roustam Temirgaliev told the Russian news agency Ria Novosti.

NATO hit back on Thursday at accusations by Moscow that the Western alliance was in violation of international law and accused Russia of fomenting "propaganda and disinformation" over the crisis in Ukraine.

"No, of course we haven't violated the Rome Declaration and I'm actually surprised that Russia can claim that NATO has violated its commitments because Russia is violating every principle and international commitment it has made," NATO Secretary General Anders Fogh Rasmussen said.

Russian Foreign Minister Sergei Lavrov on Thursday said there was a large amount of evidence to contradict claims by Kiev that Ukraine's former authorities used snipers to kill scores in February.

The version of events announced by Kiev Thursday "contradicts huge amounts of evidence that proves the contrary," Lavrov said, mentioning a leaked telephone call in which the European Union's foreign affairs chief Catherine Ashton discussed the possible involvement of the Ukrainian opposition in the shootings.