Kyiv. The United States and some other Western embassies in Kyiv said that they would stay closed Wednesday for security reasons, with the American delegation saying it had received a warning of a potentially significant Russian air attack on the Ukrainian capital.

The precautionary step came after Russian officials promised a response to President Joe Biden’s decision to let Ukraine strike targets on Russian soil with US-made missiles -- a move that angered the Kremlin.

The US Embassy said its closure and attack warning were issued in the context of ongoing Russian missile and drone attacks on Kyiv and anticipated a quick return to regular operations.

The Italian and Greek embassies also shut to the public for the day, but the UK government said that its embassy remained open.

The war, which reached its 1,000-day milestone on Tuesday, has taken on a growing international dimension with the arrival of North Korean troops to help Russia on the battlefield -- a development which US officials said prompted Biden’s policy shift.

Russian President Vladimir Putin subsequently lowered the threshold for using his nuclear arsenal, with the new doctrine announced Tuesday permitting a potential nuclear response by Moscow even to a conventional attack on Russia by any nation that is supported by a nuclear power.

That could potentially include Ukrainian attacks backed by the US.

Western leaders dismissed the Russian move as an attempt to deter Ukraine’s allies from providing further support to Kyiv, but the escalating tension weighed on stock markets after Ukraine used American-made ATACMS longer-range missiles for the first time to strike a target inside Russia.

Western and Ukrainian officials say Russia been stockpiling powerful long-range missiles, possibly in an upcoming effort to crush the Ukrainian power grid as winter settles in.

Military analysts say the US decision on the range over which American-made missiles can be used isn't expected to be a game-changer in the war, but it could help weaken the Russian war effort, according to the Institute for the Study of War, a Washington think tank.

“Ukrainian long-range strikes against military objects within Russia’s rear are crucial for degrading Russian military capabilities throughout the theater," it said.

Meanwhile, North Korea recently supplied additional artillery systems to Russia, according to South Korea. It said that North Korean soldiers were assigned to Russia’s marine and airborne forces units and some of them have already begun fighting alongside the Russians on the front lines.

Ukraine struck a factory in Russia’s Belgorod region that makes cargo drones for the armed forces in an overnight attack, according to Andrii Kovalenko, the head of the counterdisinformation branch of Ukraine’s Security Council.

He also claimed Ukraine hit an arsenal in Russia’s Novgorod region, near the town of Kotovo, located about 680 kilometers (420 miles) behind the Ukrainian border. The arsenal stored artillery ammunition and various types of missiles, he said.

It wasn't possible to independently verify the claims.

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