KYIV (Reuters) - The United States shut its embassy in Kyiv on Wednesday due to "specific information of a potential significant air attack" and told its citizens in Ukraine to be ready to swiftly seek shelter.

The unusual warning came a day after Ukraine used U.S. ATACMS missiles to strike Russian territory, taking advantage of newly granted permission from the outgoing administration of U.S. President Joe Biden on the war's 1,000th day.

"Out of an abundance of caution, the embassy will be closed, and embassy employees are being instructed to shelter in place," the U.S. Department of State Consular Affairs said in a statement on the U.S. embassy's website.

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

Russia pounded the Ukrainian power grid with 120 missiles and 90 drones on Sunday, causing damage to the power system and killing seven people in an attack that renewed fears over the durability of the hobbled energy network.

The embassy in Kyiv urged U.S. citizens in Ukraine to have reserves of water, food and other essentials such as required medications for the event of a "possible temporary loss of electricity and water" caused by Russian strikes.

"Persistent Russian attacks targeting civilian infrastructure throughout Ukraine may result in power outages, loss of heating, and disruption of municipal services," it said.

Andriy Kovalenko, head of the Ukrainian Security Council's Centre for Countering Disinformation, said Russia was ready to conduct more airstrikes, as he commented on the U.S. statement.

"Let me remind you that the Russians have been stockpiling missiles for a series of attacks on Ukraine for months. This includes Kh-101 missiles, which they continue to produce, as well as Kalibrs and ballistics," he said.

Russia had been warning the West for months that if Washington allowed Ukraine to fire U.S., British and French missiles deep into Russia, Moscow would consider those NATO members to be directly involved in the war in Ukraine.

Russian President Vladimir Putin said in October that Moscow was working on different ways to respond if Washington approved Ukrainian strikes with U.S.-made weapons deep into Russia.

On Tuesday, Putin lowered the threshold for a nuclear strike in response to a broader range of conventional attacks, with nuclear risks rising amid the highest tensions between Russia and West in more than half a century.

(Reporting by Lidia Kelly, Anastasiia Malenko, Kanjyik Ghosh and Tom Balmforth; Editing by Christopher Cushing, Himani Sarkar, Philippa Fletcher)