The United States is "keeping all our options open" when it comes to Yemen's Huthi rebels, the national security advisor said Monday, amid reports the outgoing Trump administration could tag the group as terrorists.

Speaking to reporters during a visit to the Philippines, Robert O'Brien criticised the Iran-backed Huthis of failing to engage in a "good-faith peace process" to end the conflict.

The Huthis are at the centre of a flurry of diplomacy as the Trump administration, which has made isolating its arch-foe Tehran a centrepiece of its regional policy, enters its final weeks.

Washington was monitoring the situation "very closely", said O'Brien in response to a question on whether the US would designate the Huthis a "terrorist organisation".

"We are constantly considering whether and who and how we should designate terrorist organisations," O'Brien said. 

"President Trump is still the president of the United States for the next 50 days and this will be something that is certainly on the agenda and we will have to see how that plays out," he added. 

"Right now we encourage the Huthis to expel the Iranians, to stop attacking neighbours and stop attacking people within Yemen and engage in a good-faith peace process with the other stakeholders in Yemen."

The rebel group controls the capital Sanaa and much of the north after a grinding five-year war with government forces that has created the world's worst humanitarian crisis.

The government is supported by a Saudi-led coalition, assisted by Western powers including the US.

The Huthis have reacted angrily to the prospect of the US designation, saying Trump had no right to make the ruling after failing to win a second term. 

The possibility has also alarmed humanitarian groups who say it could cripple aid delivery and tip the country into famine.