Yemen's Huthis were defiant Monday after the United States moved to brand the Iran-backed rebels as terrorists, a last-minute move in defiance of aid groups who fear it will tip the country into famine.

Unless Congress blocks the decision, the Huthis will be blacklisted on January 19 -- one day before the inauguration of President-elect Joe Biden, whose aides had hoped to mount a fresh push to end Yemen's devastating six-year-old war.

"These policies represent a crisis in thinking and are to be condemned, and we have the right to respond," Huthi political commander Mohamed Ali al-Huthi said in a tweet.

"The Yemeni people don't care about any designation from (US President Donald) Trump's administration as it is a partner in killing Yemenis and starving them."

The decision announced by US Secretary of State Mike Pompeo could complicate Biden's promised efforts to restart diplomacy with Iran and to reassess Washington's alliance with Saudi Arabia, which has led a bloody offensive in its impoverished southern neighbour.

"The designations are intended to hold Ansar Allah accountable for its terrorist acts, including cross-border attacks threatening civilian populations, infrastructure and commercial shipping," Pompeo said  Sunday, using the official name of the Huthi movement.

The Huthis have "led a brutal campaign that has killed many people, continues to destabilise the region and denies Yemenis a peaceful solution to the conflict in their country".

Pompeo also designated as terrorists three leaders of the movement, including their chief Abdul Malik al-Huthi.

He pointed to a December 30 attack on an airport in Yemen's second city Aden, which killed 26 people and was blamed by the Saudi-backed government on the Huthis.

- Fears of aid groups -

The Huthis control much of Yemen, including the capital Sanaa, and are already under US sanctions. 

The designation is expected to scare away outside actors from many transactions with Huthi authorities, including bank transfers and buying food and fuel, for fear of US prosecution.

Aid groups have warned Pompeo against the blacklisting, saying they have no option but to deal with what is the de facto government in northern Yemen.

"The US government must ensure that any sanctions do not block food, fuel and medicines from entering a country already in the middle of a full-blown humanitarian catastrophe," the Norwegian Refugee Council's Yemen director Mohamed Abdi said in a statement.

Pompeo insisted that the State Department was aware of the concerns and was "planning to put in place measures" to reduce the impact on humanitarian work and imports into Yemen.

Trump's administration has been ramping up pressure on Iran in its final days, hoping to make it more difficult logistically and politically for Biden to ease sanctions as he seeks a return to a nuclear deal.

US officials and analysts say Iran has armed the Huthis, but some experts question the extent of cooperation and see Tehran primarily as interested in bogging down Saudi Arabia, whose brutal air campaign has included strikes on civilian targets.

Tens of thousands of people, mostly civilians, have been killed and millions displaced in Yemen's war, with most of the nation dependent on some form of aid to survive.

Analysts warn of disastrous consequences after the UN World Food Programme said in December that malnutrition had reached record levels, narrowing the window of opportunity to prevent a famine.

Peter Salisbury, senior Yemen analyst at the International Crisis Group, said the designation "risks collectively punishing all Yemenis by precipitating a famine while doing little to hurt the Huthis other than pushing them closer to Iran".

"If the impact of this designation is half as bad as has been predicted it is millions of ordinary Yemenis who are struggling to eat who will pay the price, while already distant prospects of peace slip away."

- Ignoring Congressional warnings -

Under US law, Congress has seven days in which it can object to a designation of a foreign terrorist group.

But it will likely be focused on other issues, as the House of Representatives looks at impeaching Trump for a second time after he encouraged a mob that stormed the Capitol last week to disrupt a session that certified Biden's election win.

Lawmakers from Biden's Democratic Party have told Pompeo the designation would both jeopardise aid and peace efforts.

"This designation would almost certainly prevent the critical delivery of food, medical supplies and other items necessary to combat both Covid-19 and famine," three senators said in a joint appeal last year.

In an open letter to Pompeo last month, retired US diplomats, including six former ambassadors to Yemen, also doubted that the Huthis met the legal definition of a terrorist group.

"In fact, designating the Huthi movement will be perceived as politically motivated and will undermine the credibility of US counterterrorism programs and policies, which serve as a first line of our nation's defence," they wrote.