Ten Russian soldiers were detained in the Donetsk region of eastern Ukraine, that country’s Security Service said Tuesday, as tensions simmered over the conflict between Ukrainian forces and pro-Russian rebels.

The Russian soldiers were captured with documents and weapons on them, the Security Service said.

Russia has repeatedly denied claims by Kiev that it has sent troops and weapons over the border into Ukraine, where the Ukrainian military is fighting pro-Russian rebels.

Russia’s state-run RIA Novosti news agency cited a source in the Russian Defense Ministry as saying the soldiers had been patrolling the border and “most likely crossed by accident” at an unmarked point.