KYIV, Ukraine (AP) — The Biden administration said Wednesday that it will give Ukraine antipersonnel mines to help it slow Russia’s battlefield advances, marking the second major shift on U.S. military support for Kyiv in days.

After allowing Ukraine to use longer-range American missiles to launch strikes deeper into Russia, U.S. Defense Secretary Lloyd Austin said the shift in Washington’s policy on antipersonnel land mines for Ukraine was needed to counter changing Russian tactics.

The war, which reached its 1,000-day milestone on Tuesday, has largely been going Russia’s way. Moscow’s bigger army is slowly pushing Ukraine’s forces backward in the eastern Donetsk region, while Ukrainian civilians are being maimed and killed by Russian drones and missiles often fired from inside Russia.

WATCH NOW: On 1,000th day of war, Ukraine hits targets inside Russia with U.S.-made missile system

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

The announcement comes two months before Donald Trump replaces Joe Biden in the White House. Trump has pledged to swiftly end the war and has criticized the amount the U.S. has spent on supporting Ukraine. Biden administration officials say they are determined to help Ukraine as much as possible before he leaves office.

The U.S. steps taken to help Ukraine’s military — along with unconfirmed reports Wednesday of Ukraine firing a certain British cruise missile at Russia for the first time — were likely to vex Moscow.

The U.S. and some other Western embassies in Kyiv closed for the day Wednesday in response to the threat of a potentially major Russian aerial attack on the Ukrainian capital.

Humanitarian groups have long criticized the use of antipersonnel mines because they present a lingering threat to civilians. Amnesty International called the U.S. decision “reckless” and a “deeply disappointing setback.” And Norway’s foreign minister, Espen Barth Eide, called it “very problematic” because Ukraine is a signatory to an international convention opposing the use of land mines.

Austin pointed out that Ukraine already makes its own antipersonnel mines, and that the U.S. has been providing Ukraine with anti-tank mines. He also tried to allay concerns about the new mines the U.S. is giving Kyiv, saying they are not persistent, meaning troops can control when they would self-detonate.

“That makes it far more safer eventually than the things that they are creating on their own,” Austin said.

The mines are are electrically fused and powered by batteries so that when the battery runs out, they won’t detonate. They can become inert in anywhere from four hours to two weeks.

Russia already uses land mines in Ukraine, but those don’t become inert over time.

The United States also sought commitments from the Ukrainians on the use of the mines to limit harm to innocent civilians, according to a U.S. official who spoke on condition of anonymity to discuss internal deliberations. Ukraine would use the mines in its own country and would not put them in civilian populated areas.

The mines are contained in a $275 million package of new military assistance from the Biden administration, according to a different U.S. official who spoke on condition of anonymity because the aid package hasn’t been formerly announced. Also included in the package are High Mobility Artillery Rocket Systems, or HIMARS, as well as 155 mm and 105 mm artillery rounds, Javelin anti-armor munitions, and other equipment and spare parts.

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

Britain had been quietly pressing the U.S. to ease restrictions on how Western-supplied missiles are used, and unconfirmed news reports Wednesday said Ukraine had fired British-made Storm Shadow cruise missiles at Russia for the first time. British and Ukrainian officials didn’t confirm the reports.

Officials with France’s military and president’s office, meanwhile, declined to say whether Ukraine is using French long-range SCALP missiles to strike targets in Russia, citing France’s military secrecy policy. French President Emmanuel Macron has been pushing for such a step for months.

After the Biden administration allowed Ukraine to attack Russia with longer-range American-made ATACMS missiles, Russian President Vladimir Putin 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.

The American diplomatic mission in Kyiv said Wednesday that it had received a warning about a potentially significant Russian air attack on the Ukrainian capital and was staying closed for the day. It anticipated a quick return to regular operations. The Spanish, Italian and Greek embassies also closed for the day, but the U.K. government and France said that their embassies remained open.

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 fired ATACMS missiles for the first time at 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 approaches.

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, 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 counter-disinformation 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.

Baldor reported from Washington. Associated Press reporters Tara Copp in Washington, Jill Lawless in London and Angela Charlton in Paris contributed to this report.