Russian President Vladimir Putin has ordered the tightening of the country’s borders after a swarm of drones flew deep inside the country on Wednesday.

The drones, believed to have been launched from a nearby country, flew as far as the city of Voronezh, around 600 miles south of Moscow, according to the Russian Defense Ministry. The drones were reportedly carrying explosives, prompting Putin to order the security forces to take all necessary measures to strengthen the country’s borders.

The Defense Ministry also said that the drones had been shot down by the Russian military and that the incident was being investigated.

The incident comes at a time of heightened tensions between Russia and its neighbors, a situation that has been further exacerbated by the ongoing conflict in Ukraine.

Though no group has yet claimed responsibility for the drone attack, some analysts have suggested that it could be a sign of escalating tensions between Russia and its neighbors.

The incident is also likely to further strain the already fragile relationship between Russia and the West, with the United States and other Western countries calling for increased sanctions against Russia over its involvement in the Ukraine crisis.