US President Joe Biden has approved provision of anti-personnel land mines to Ukraine, a US official told Reuters, a step that could help slow Russian advances in its east, especially when used along with other munitions from the United States.

The United States expects Ukraine to use the mines in its own territory, though it has committed not to use them in areas populated with its own civilians, the official said. The Washington Post first reported the development.

The office of Ukraine’s President Volodymyr Zelenskiy, the Ukrainian defence ministry, the Russian defence ministry and the Kremlin did not immediately respond to Reuters’ requests to comment.

The United States has provided Ukraine with anti-tank mines throughout its war with Russia, but the addition of anti-personnel mines aims at blunting the advance of Russian ground troops, the official added, speaking on condition of anonymity.

The US mines differ from Russia’s as they are “non-persistent,” and become inert after a preset period, the official said. They require a battery to detonate, and will not explode once the battery runs out.

On Tuesday, Ukraine used US ATACMS missiles to strike into Russian territory, taking advantage of newly granted permission from Biden’s outgoing administration on the war’s 1,000th day.

Moscow said the use of ATACMS, the longest-range missiles Washington has yet supplied to Ukraine, was a clear signal the West wanted to escalate the conflict.

On Tuesday, Russian President Vladimir Putin lowered the threshold for a nuclear strike in response to a broader range of conventional attacks.

The move followed months of warnings to the West that if Washington allowed Ukraine to fire US, British and French missiles deep into Russia, Moscow would consider those NATO members to be directly involved in the war in Ukraine.