The United States has shut down its embassy in Ukraine’s capital and ordered U.S. citizens in Kyiv to immediately seek shelter, citing a potential air attack by Russia. The warning came as Ukraine for the first time used U.S.-supplied long-range ATACMS missiles to strike targets deep inside Russia. This follows President Vladimir Putin’s warning that Russia had lowered its threshold for the use of nuclear weapons, in response to escalating U.S. and European support for Ukrainian counterattacks on Russian soil.