Yemen's Houthi group and Saudi-backed government who have been at war for almost six years began fresh U.N.-backed negotiations on a prisoner exchange on Sunday, United Nations and Yemeni officials said.

The meeting in Jordan comes days after the United States designated the Iran-aligned Houthis as a foreign terrorist organisation, a move the United Nations had warned could undermine peace efforts and worsen Yemen's humanitarian crisis.

A U.N.-chartered plane carried four Houthi officials from Sanaa to Amman on Saturday. The government also sent four representatives, according to Mohammad Fadayel, the head of the government's prisoners committee.

The talks aim to free 300 prisoners, including high-ranking officials like the brother of President Abd-Rabbu Mansour Hadi, whose internationally recognised government was ousted by the Houthis, sources familiar with the matter said.

"The meetings started on Sunday morning," Ismini Palla, spokeswoman of the office of U.N. special envoy Martin Griffiths, told Reuters, adding that he had opened the talks.

The talks are part of confidence-building measures aimed at restarting peace negotiations last held in Sweden in December 2018, when the two parties agreed to exchange 15,000 detainees split between the Iran-aligned Houthi movement and the Saudi-led coalition that has been battling the group since 2015.

Around a 1,000 prisoners were exchanged last year.

Griffiths, in a statement on Sunday, urged the parties to discuss and agree on names "beyond the Amman meeting lists to fulfill their Stockholm commitment of releasing all conflict-related detainees as soon as possible".

The administration of new U.S. President Joe Biden said on Friday it has initiated a review of the designation, which went into effect on Jan. 19 ahead of Biden entering the White House.

The Houthis' chief negotiator has told Reuters the group would not walk away from talks.

The conflict, largely seen in the region as a proxy war between Saudi Arabia and Iran, has killed more than 100,000 people and led to what the U.N. describes as the world's largest humanitarian crisis with millions facing starvation.