Yemen's Huthi rebels, who control much of the north of war-ravaged country, warned Tuesday they would respond to any action against them following their US classification as "terrorists".

"We are ready to take all necessary measures against any hostile act," the Iran-backed Huthis said in a statement.

Outgoing US President Donald Trump's administration announced the last-ditch move on January 11, with just days to go before his successor Joe Biden takes over.

The United Nations and aid groups have warned it risks worsening an already dire humanitarian crisis.

Yemen's war has killed tens of thousands, displaced millions and, according to the UN, left 16 million on the brink of famine.

The designation is expected to halt many transactions with Huthi authorities, including bank transfers and payments to medical personnel and for food and fuel, due to fears of US prosecution.

In turn, the Huthis called the US "criminal terrorists".

The designation comes on the eve of the inauguration of Biden, whose aides had hoped to mount a fresh push to end Yemen's six-year war.

The Huthis are facing a bloody offensive led by US ally Saudi Arabia, with millions in the country depending on aid to survive.

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