Russia on Wednesday accused Ukraine of attempting to attack critical infrastructure in Crimea and said two Russians were killed in related clashes.
Russia's Federal Security Service (FSB), a successor agency of the Soviet KGB, said it detained several men who were in possession of 20 explosive devices and are believed to have been working for Ukraine's military intelligence agency.
Two Russians - a soldier and a member of the FSB - were killed while trying to detain the Ukrainian military cell, the FSB said in a statement on its website.
Ukraine quickly denied the allegation. "Ukraine is not and will not retake its territory with force," Yury Tandit of Ukraine's Security Service told the Interfax news agency.
The claims follow a recent surge in fighting in eastern Ukraine, with last month reported as the most deadly in more than a year.
Russia occupied and annexed Ukraine's southern Crimea region two years ago in response to Kiev ousting its pro-Russian president amid mass protests calling for closer ties with the West.
Soon after Russia annexed Crimea, a pro-Russian separatist rebellion erupted in Ukraine's two eastern-most regions. Ukraine has accused Russia of fuelling the conflict with weapons and soldiers for the separatists, allegations Moscow has denied.