The US decision to send anti-personnel landmines to Ukraine – in a major policy shift – was triggered by a change in Russian battlefield tactics favouring infantry over mechanised units, US Defence Secretary Loyd Austin said on Wednesday.

"They don't lead with their mechanised forces any more," Austin told reporters while on a visit to Laos. "They lead with dismounted forces who are able to close and do things to kind of pave the way for mechanised forces."

The Ukrainians "have a need for things that can help slow down that effort on the part of the Russians."

President Joe Biden's reversal of his previous curbs on US landmines comes just days after Washington gave Ukraine the green light to use US-made long-range missiles on targets within Russia, as the outgoing administration aims to give Kyiv the upper hand before President-elect Donald Trump enters office.

Biden in 2022 said the United States would mostly ban its use of landmines, at the time specifically drawing a contrast with Russia's use of the weapons in Ukraine.

The reversal comes amid concerns about the incoming Trump administration's lack of support for Kyiv.

Trump is entering office having repeatedly criticised US assistance for Ukraine, claiming he could secure a ceasefire within hours – comments that have triggered fears in Kyiv and Europe about Ukraine's ability to withstand the Russian attacks without American support.

Both Russia and the United States – neither of which are signatories to the UN Mine Ban Treaty – have been criticised for their past use of anti-personnel mines in the past. Ukraine is a signatory of the treaty.

Austin said the US-supplied mines would be so-called "nonpersistent" mines that can self-destruct or render themselves inactive after losing battery charge – in theory limiting the risk to civilians.

Both Moscow and Kyiv are jockeying to secure battlefield advantage before Trump assumes office next January.

This week, Kyiv fired US-supplied ATACMS missiles at Russian territory for the first time.

On Tuesday, Russian President Vladimir Putin signed a decree lowering the threshold for when Russia could use nuclear weapons. (AFP)