Russian President Vladimir Putin accused Ukrainian forces of a “criminal” incursion into Crimea, in an escalation of the conflict between the two countries that began when the peninsula was annexed by Moscow in early 2014.

Russia’s Federal Security Service (FSB) said on Wednesday that it had foiled “terrorist acts” prepared by Ukrainian military intelligence against infrastructure in the territory, with the aim of disrupting Russia’s parliamentary elections due on 18 September. Kiev has denied the allegations.

The FSB, the successor organisation to the KGB, claimed that one of its officers and a Russian soldier had been killed while dealing with the Ukrainian plot last weekend. The FSB said the soldier died in an exchange of fire with the Ukrainian army across the border that now separates Crimea from the rest of Ukraine — a level of fighting between the two militaries not seen even during the annexation. Moscow said it had arrested several people, including a Ukrainian military intelligence officer.