KYIV, Ukraine — The Biden administration will allow Ukraine to use American-supplied antipersonnel land mines to help it slow Russia’s battlefield progress in the war, the U.S. defense secretary said Wednesday, in Washington’s second major policy shift in a week after its decision to let Ukraine strike targets on Russian soil with longer-range U.S.-made missiles.

The war, which reached its 1,000-day milestone on Tuesday, has largely been going Russia’s way in recent months. Russia’s bigger army is slowly pushing Ukraine’s outnumbered army backward in the eastern Donetsk region, while Ukrainian civilians have repeatedly been clobbered by Russian drones and missiles often fired from inside Russia.

The U.S. and some other Western embassies in Kyiv stayed closed Wednesday after a threat of a major Russian aerial attack on the Ukrainian capital.

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Change in US Decision Follows Changes in Russian Tactics

U.S. Defense Secretary Lloyd Austin said the change in Washington’s policy on antipersonnel land mines for Ukraine follows changing tactics by the Russians.

Russian ground troops are leading the movement on the battlefield, rather than forces more protected in armored carriers, so Ukraine has “a need for things that can help slow down that effort on the part of the Russians,” Austin said during a trip to Laos.

Antipersonnel land mines have long been criticized by charities and activists because they present a lingering threat to civilians. Austin countered that argument.

“The land mines that we would look to provide them would be land mines that are not persistent, you know, we can control when they would self-activate, self-detonate and that makes it far more safer eventually than the things that they are creating on their own,” Austin said.

Russia has already been using land mines in Ukraine.

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Nonpersistent land mines generally require batteries, so overtime they become unable to detonate, making them safer for innocent civilians than those that remain deadly for years.

Austin noted that Ukraine is currently manufacturing its own antipersonnel land mines.

The U.S. already provides Ukraine with antitank land mines. Russia has routinely used land mines in the war, but those do not become inert overtime.

The war has taken on a growing international dimension with the arrival of North Korean troops to help Russia on the battlefield — a development which U.S. officials said prompted Biden’s policy shift on longer-range missiles and which angered the Kremlin.

Putin Lowers Threshold for Using Nuclear Arsenal

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 U.S..

Austin’s announcement Wednesday was likely to further vex Russia.

The American diplomatic mission in Kyiv said it had received a warning of a potentially significant Russian air attack on the capital and was staying shut for the day. It anticipated a quick return to regular operations.

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The Spanish, Italian and Greek embassies also shut to the public for the day, but the U.K. government and France said that their embassies remained open.

Western Leaders Dismiss Russian Reaction

Western leaders dismissed the Russian reaction to the U.S. missile decision 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 U.S. 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.