KYIV, Ukraine — Ukraine fired six American-supplied longer-range missiles at Russia’s Bryansk region, Moscow said Tuesday, in what would be Kyiv’s first use of the weapons inside Russia in 1,000 days of war.

The reported use of the Army Tactical Missile System, known as ATACMS, came as Russian President Vladimir Putin formally lowered the threshold for using nuclear weapons, opening the door to a potential nuclear response by Moscow to even a conventional attack by any nation supported by a nuclear power. That could include Ukrainian attacks backed by the U.S.