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.

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.

Nonpersistent land mines generally require batteries, so over time 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.