• The U.S. closed its embassy in Kyiv on Wednesday, warning that it has "received specific information of a potential significant air attack" amid soaring tensions with Russia.
• The U.S. Embassy said in a statement that it was closing the building "out of an abundance of caution" and instructed embassy employees to shelter in place.
• "The U.S. Embassy recommends U.S. citizens be prepared to immediately shelter in the event an air alert is announced," it added.

The U.S. closed its embassy in Kyiv on Wednesday, warning that it has "received specific information of a potential significant air attack" amid soaring tensions with Russia.

The U.S. Embassy said in a statement that it was closing the building "out of an abundance of caution" and instructed embassy employees to shelter in place.

"The U.S. Embassy recommends U.S. citizens be prepared to immediately shelter in the event an air alert is announced," it added.

Asked to comment on the embassy's closure on Wednesday, the Kremlin's press secretary, Dmitry Peskov, said only that the move was "about the course of the Special Military Operation," as Russia describes its ongoing war against Ukraine.

Air alerts in Kyiv are a common occurrence as the Ukrainian capital suffers frequent drone and missile attacks by Russia. But the latest alert comes amid rising tensions between Moscow and Washington, after reports that the White House on Sunday gave Ukraine permission to use U.S.-made long-range missiles to attack targets on Russian territory.

On Tuesday, Russia's Defense Ministry said it had intercepted five U.S.-made ATACMS missiles, and destroyed one other, which had been used by Ukraine to attack its border region of Bryansk. Ukraine has not officially commented on carrying out an attack using long-range American-made missiles. 

The Pentagon also declined to comment when asked about the development.

The attack drew a furious reaction from Moscow, however, with Foreign Minister Sergey Lavrov calling the move an "escalation" by the West.

The Kremlin has repeatedly warned the West against allowing Ukraine to use its long-range weapons against targets on Russian soil. On Tuesday, President Vladimir Putin approved amendments to Russia's nuclear doctrine, shifting the parameters in which Moscow can use nuclear weapons.

The updated document now states that any aggression against Russia by a non-nuclear state that is supported by a nuclear power will be considered as a joint attack.

It also outlines that the Kremlin may use nuclear weapons in the event of a critical threat to its sovereignty and territorial integrity — and that of its ally, Belarus — and that the launch of ballistic missiles against Russia would be seen among the conditions that could warrant a nuclear response.

Russia's foreign intelligence chief, Sergei Naryshkin, said Wednesday that the West "understands that the adjustments announced by Putin largely devalue efforts by the United States and NATO to inflict a 'strategic defeat' on our country while the planned expansion of the list of grounds for the use of nuclear weapons effectively excludes the possibility of victory over the Russian Armed Forces on the battlefield," he said in an interview with the National Defense outlet, in comments translated by NBC News.

"Our enemies are forced to admit that the determination of the Russian President to firmly defend the fundamental interests of the country by all available means narrows the room for maneuver for Washington and Brussels. Attempts by individual NATO allies to participate in providing for possible long-range strikes with Western weapons deep into Russian territory will not go unpunished," he added.