Ukrainian troops have launched an attack against a Russian border region with six US long-range missiles, Moscow has said. It comes after Vladimir Putin approved an updated doctrine lowering the threshold for the use of nuclear weapons.

The Army Tactical Missile System, or ATACMS, are short-range ballistic missiles that can travel a lot farther than the weapons previously at Kyiv’s disposal.

The strike, against an ammunition depot, comes days after the U.S. authorized the use of ATACMS on Russian territory.