KIEV: Ukraine's security service said Thursday it had arrested an insurgent group involving three Russian nationals that had stockpiled bombs and planned to carry out “terrorist attacks”, in raids that cost the life of a special forces officer.

“The insurgents planned to stage terrorist acts in Kiev and Kharkiv to destabilise the situation in the country,” a spokeswoman for the SBU security service, Olena Gitlyanska, said on Facebook.

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A special forces officer and a member of the insurgent group, who opened fire, were killed in operations in those cities late Wednesday and early Thursday, she said.

Another special forces officer was wounded but his condition was stable, she said.

“In one apartment alone, where active resistance was put up and where the leader of the insurgent group was liquidated, eight improvised explosive devices were found,” another SBU official, Oleksandr Tkachuk, told reporters.

“The Russian nationals were detained in another apartment. Twenty three more explosive devices were found there,” he said.

Gitlyanska added officials had also found automatic weapons, more than 40 grenades and falsified documents.

Tkachuk did not rule out that the detained Russian nationals were linked to Russian special services.

Kiev and the West have accused Russia of fuelling an insurgency in eastern Ukraine and sending in regular troops across the border in a conflict that has claimed more than 9,000 people since April, 2014.

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Russia has denied the claims. President Vladimir Putin's spokesman Dmitry Peskov told reporters on Thursday he did not have any details about the reported operation in Ukraine.

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Ukraine's population is divided in loyalties between Russia and Europe, with much of western Ukraine advocating closer ties with the European Union while eastern and southern regions look to Russia for support. Crimea, a semi-autonomous region of Ukraine, is mainly Russian-speaking.

In Crimea, the pro-Russian prime minister who took office after gunmen seized the regional Parliament claimed control of the military and police there and asked Putin for help in keeping peace, sharpening the discord between the two neighboring Slavic countries.

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Crimea only became part of Ukraine in 1954 when Soviet leader Nikita Khrushchev transferred jurisdiction from Russia, a move that was a mere formality when both Ukraine and Russia were part of the Soviet Union. The Soviet breakup in 1991 meant Crimea landed in an independent Ukraine.