Russian forces killed a group of five saboteurs who breached the country’s southwest border from Ukraine on Monday, news agencies quoted the military as saying, in an accusation that Kiev dismissed as the latest in a series of fakes.

The accusations escalated already soaring tensions over a Russian military buildup. Kiev and the West fear that a border incident near eastern Ukraine could be used as a pretext for Moscow to attack its neighbour. Russia denies such plans. Russia’s rouble fell to its weakest against the dollar since January 27 on the reports, which quoted a statement from Russia’s Southern Military District.

It said that a Federal Security Service border patrol around 0300 GMT identified a sabotage and reconnaissance group that had breached the border in Rostov region, prompting it to call troops for backup. A clash ensued, it said.

“Two Ukrainian army infantry fighting vehicles crossed the state border into the Russian Federation to evacuate the sabotage group from the territory of Ukraine,” it said. “As a result of the engagement, five members of the sabotage and reconnaissance group who violated the Russian border were eliminated,” it said.

The group was detected near the village of Mityanskaya in Russia’s Rostov region, it said, adding that there were no casualties on the Russian side. Ukraine dismissed the account as false information.

“Another fake. We don’t have people there, nor could they be (there),” Ukraine’s military said.