Mohamed Qahtan is the leader of the Sunni Islamist Islah party, which is aligned with the Saudi Arabia-backed government, and he has been held incommunicado by the Iranian-backed Houthis since 2015, the United Nations said.

The office of UN special envoy Hans Grundberg convened a meeting in Oman with the International Committee of the Red Cross over the weekend to facilitate talks centred on a prisoner exchange, in accordance with the 2018 Stockholm Agreement, UN spokesman Stephane Dujarric said.

The two Yemeni sides agreed to meet to discuss the final list of who will be released and the details of Qahtan's release, Dujarric said, without providing details on the potential agreement.

Qahtan's release has been a point of contention for years, Dujarric said.

In addition to Qahtan, the Houthi rebels have detained at least 45 Yemeni employees of UN agencies, diplomatic missions, private companies and other organisations.

Grundberg urged the parties to agree on more releases and also repeated a demand for the immediate and unconditional release of UN personnel, aid workers and others detained in the Houthi-held capital of Sanaa who continue to be held incommunicado, Dujarric said.

At a UN Security Council meeting on Monday, US deputy ambassador Stephanie Sullivan called for the detainees' immediate release.

“The detention of UN and NGO staff directly and negatively affects the ability of these organisations to provide humanitarian aid,” she said.

Yemen has been engulfed in civil war since 2014, when the Houthis seized much of northern Yemen and forced the internationally recognized government to flee from Sanaa. A Saudi-led coalition intervened the following year in support of government forces, and in time the conflict turned into a proxy war between Saudi Arabia and Iran.

The war has killed more than 150,000 people, including fighters and civilians, and created one of the world's worst humanitarian disasters.

Since November, the Houthis have attacked shipping in nearby waters, saying they aim to pressure Israel to halt its war in Gaza. The Houthis have targeted more than 60 vessels with missiles and drones, killing a total of four sailors.

A US-led airstrike campaign has targeted the Houthis since January, killing at least 16 people and wounding 42 others, the rebels say.