WASHINGTON: The outgoing Trump administration's decision to classify Yemen's Huthi rebels as terrorists will only cause more suffering for the people of that war-torn nation, Joe Biden's nominee for national security advisor said Saturday.

The rebels control much of Yemen and have faced an offensive from US ally Saudi Arabia, with millions in Yemen depending on aid to survive.

Designating the Huthis a terrorist group is expected to halt many transactions with Huthi authorities, including bank transfers, paying medical personnel and for food and fuel, due to fears of US prosecution.

"Huthi commanders need to be held accountable, but designating the whole organization will only inflict more suffering on Yemeni people and impede diplomacy critical to end the war," Biden's pick for national security advisor, Jake Sullivan, tweeted.

The designation is set to come into force on January 19 -- the eve of the inauguration of Biden, whose aides had hoped to mount a fresh push to end Yemen's six-year war.

It is also seen as complicating the incoming US leader's promised efforts to restart diplomacy with Iran, which has links to the Huthis.

The terrorist classification has drawn criticism from aid groups, the European Union and many others over fears it will exacerbate the already dire humanitarian crisis in Yemen.

"What is the likely humanitarian impact? The answer is a large-scale famine on a scale that we have not seen for nearly 40 years," Mark Lowcock, the UN undersecretary general for humanitarian affairs, said Thursday.

Lowcock said exemptions to allow aid agencies to deliver supplies, as suggested by Washington, would not be sufficient to avoid a famine, adding "what would prevent it? A reversal of the decision."