Embassy in the capital said it had received "specific information" about a "potential significant air attack" planned for that day."Out of an abundance of caution, the Embassy will be closed, and Embassy employees are being instructed to shelter in place," the statement read.The announcement came in the wake of what appeared to be Ukraine's first successful strike of a military target inside Russia using the U.S.-supplied ATACMS missiles.After depicting such a move as crossing another "red line" the Kremlin had drawn, Russian President Vladimir Putin said his country would respond. A mass attack using cruise and ballistic missiles, as well as drones, was the most expected outcome.Throughout the full-scale invasion of Ukraine, the Kremlin has repeatedly laid down red lines, the crossing of which it has said could provoke various responses, sometimes invoking the threat of nuclear weapons.In an attempt to deter Ukraine's Western allies from further arming Kyiv, the Kremlin issued warnings related to the supply of Patriot air defense systems, F-16 fighter jets, and ATACMS to be used in occupied areas of Ukraine, among others.Despite crossing multiple red lines on multiple occasions, Russia has yet to follow through with any of its threats.What has spooked some observers about the latest crossing is that it happened just hours before Putin approved the updated principles of Russia's nuclear deterrence policy.One of the provisions of the new policy states that "aggression against the Russian Federation and its allies by a non-nuclear state with the support of a nuclear state" could justify a nuclear strike — Ukraine using ATACMS to strike deep inside Russia would appear to qualify.Shortly after the U.S.