Russian President Vladimir Putin met with the country's security council on Thursday to discuss further security measures to secure Crimea's land border with Ukraine as well as its surrounding waters and airspace.
Russia is planning additional security measures in Crimea, the Kremlin said, after claiming that intelligence officials foiled an attempted terrorist attack on infrastructure there by Ukrainian military forces.
Russia's Federal Security Service (FSB), a successor agency of the Soviet KGB, said Wednesday it had detained several men in possession of 20 explosive devices and believed to have been working for Ukraine's military intelligence agency.
Two Russians - a soldier and a member of the FSB - were killed in clashes with the Ukrainian military cell, the FSB said in a statement on its website.
Ukrainian President Petro Poroshenko on Wednesday called Russia's allegation of a Ukrainian incursion into Crimea a "pretext for more military threats against Ukraine."
"Russian accusations that Ukraine has launched terror attacks in occupied Crimea are as cynical and insane as claims that there are no Russian troops in eastern Ukraine," Poroshenko said in a statement.
The claims follow a recent surge in fighting in eastern Ukraine, with last month reported as the deadliest 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.