Russia's Federal Security Service (FSB) said on Monday it had detained a Ukrainian man caught "red-handed" trying to steal state secrets about its small arms industry in Tula, a town that hosts a military weapons factory.
The FSB said that a criminal espionage case had been opened into the Ukrainian citizen, whom it did not name. It said he had been working for the Ukrainian intelligence services.
The Ukrainian foreign ministry did not immediately respond to a request for comment.
The FSB said that the alleged spy had tried to recruit workers at Tula's weapons factory as informers. The plant, located around 170 km (105 miles) south of Moscow, produces guided missiles as well as advanced automatic and sniper rifles for military use according to its website.
The FSB said the man had been trying to obtain state secrets regarding the latest types of guns and related secret technical documents as well as wepaons under development.
Relations between Kyiv and Moscow plummeted after Russia annexed Crimea from Ukraine in 2014 and backed a pro-Russian separatist rebellion in eastern Ukraine.
Kyiv estimates that 14,000 people have been killed in the conflict in Donbass, eastern Ukraine, since 2014.