Ukraine's SBU security service said on Monday it had detained a Russian military intelligence agent who was planning attacks on the country's largest Black Sea port of Odessa.
"(His) main task was to shake up the political situation in the Odessa region through sabotage and terrorist acts," SBU said. The agent was detained while trying to recruit someone to carry out attacks, it added.
Officials at Russia's foreign and defence ministries did not immediately respond to a request for comment on the Ukrainian statement, which did not go into details.
Kyiv, Washington and Western capitals have raised concerns about a Russian troop buildup near Ukraine and the possibility of an invasion. US and Russian officials began talks in Geneva on Monday to try to deescalate the crisis.
Moscow has denied planning an attack and accused Ukraine and NATO of fomenting tensions, demanding that the US-led alliance guarantees it will not grant Kiev membership.
Russia said last month that someone had thrown a Molotov cocktail at the Russian consulate in the Ukrainian city of Lviv and that it had formally protested over the attack, which it called "an act of terrorism".
Ukraine's ties with Russia collapsed in 2014 after Russia's annexation of Crimea and Moscow-backed forces seized territory in eastern Ukraine that Kyiv wants back. Kyiv says some 15,000 people have been killed in fighting since then.