The U.S. Embassy in Kyiv, Ukraine, issued a warning after receiving "specific information of a potential significant air attack" allegedly taking place on Wednesday.

The embassy in Ukraine's capital is temporarily closed following the alert and employees are being asked to prepare 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," the statement said.

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Employees have been asked to take the following actions:

This comes after Ukraine fired American-supplied long-range missiles into Russia on Tuesday, marking the first time for Kyiv to do so in the 1,000 days of war, which was authorized by President Biden on Sunday.

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This was not the first time the embassy has issued a warning of potential danger and a significant attack. 

A similar warning was issued around Ukraine's Independence Day on Aug. 24. 

Russian President Vladimir Putin signed a revised nuclear doctrine on Tuesday, warning that any attack on Russia supported by a country with nuclear power could be grounds for a nuclear response.

Fox News Digital's Elizabeth Pritchett contributed to this report.