US embassy in Ukraine warns of 'potentially significant air attack' that could happen in next 24 hours

Secretary of State Marco Rubio outlines the U.S. position on peace negotiations in Ukraine.

The U.S. Embassy in Kyiv issued asecurity alertSaturday warning. a "potentially significant air attack" could hit the city in the next 24 hours.

The U.S. Mission Ukraine said in a statement, "The U.S. embassy in Kyiv has received information concerning a potentially significant air attack. may occur at any time over the next 24 hours."

"The embassy, as always, recommends U.S. citizens be prepared to immediately shelter in the event an air alert is announced," the warning added.

The message follows a warning from Ukrainian President Volodmyr Zelenskyy. Russia was preparing to strike Ukraine with a hypersonic Oreshnik missile.

This is a developing story. Please check back for updates.