KYIV - Ukrainian authorities say they have arrested seven people they claim were sent by Russian security services to carry out political killings and other "terrorist" acts, including the slaying of Ukrainian intelligence agents.

Ukrainian Security Service (SBU) chief Vasyl Hrytsak made the announcement April 17, four days ahead of Ukraine's presidential runoff vote.

At a news conference, Hrytsak said the SBU thwarted "a sabotage and reconnaissance terrorist group of the Russian special services" that consisted of seven people, all of whom have been arrested.

One person who assisted the group was arrested April 17, he said, but it was not clear if that was in addition to the other seven.

Russia seized control of Ukraine's Crimean Peninsula in March 2014 and has given crucial backing to militants who hold parts of the Donetsk and Luhansk regions in a war that has killed some 13,000 people since April 2014.

Hrytsak alleged that since early 2017, the Russian security services had sent several "autonomously operating" sabotage groups into parts of Ukraine including the separatist-held section of the Donetsk region.

He said these groups were responsible for attacks including a car bombing that killed Ukrainian military intelligence officer Maksim Shapoval in June 2017 and one that missed its apparent target, also a military intelligence officer, in Kyiv earlier this month.

Investigators work at the scene of a car bomb expl FILE - Investigators work at the scene of a car bomb explosion which killed Maksim Shapoval, a high-ranking official involved in military intelligence, in Kyiv, Ukraine, June 27, 2017. FILE - Investigators work at the scene of a car bomb explosion which killed Maksim Shapoval, a high-ranking official involved in military intelligence, in Kyiv, Ukraine, June 27, 2017.

Prosecutors said at the time that the man suspected of planting that bomb, on April 4, was killed by the blast. However, Hrytsak said that the suspect, a Russian man, was alive and had given information to the Ukrainian authorities.

Hrytsak alleged that "the true organizer" of operations that included the killing of Shapoval was an officer of the Russian Federal Security Service (FSB), Dmitry Minayev.

SBU officials identified one of the seven suspects whose arrests were announced on April 17 as Timur Dzortov, who they said was deputy chief of staff to the leader of Russia's Ingushetia region, Yunus-Bek Yevkurov, in 2015-17.

There was no immediate comment from Russian officials.