Dubai (AFP)

The United States' intervention to end Yemen's long conflict has energised a once lifeless peace process, observers say, but savage fighting in the country's north has thrown up yet another barrier.

After six years of relentless fighting, the conflict that has brought Yemen to its knees appears to be at a turning point, just as the United Nations warns the country could tip into wide-scale famine at any moment.

The US has withdrawn support for the Saudi-led coalition's offensive in Yemen, and sources say its new envoy Tim Lenderking met face-to-face with the Iran-backed Huthi rebels, who unleashed the conflict when they seized the capital Sanaa.

After the freewheeling approach under former US president Donald Trump, the might of Washington's diplomacy has now swung back behind the UN pointman on Yemen, Martin Griffiths, whose unenviable job it is to get the two sides to the table.

And Saudi Arabia, with its poor rights record back in the spotlight under Joe Biden's administration, is under pressure to find a way out of the quagmire it waded into when it swooped in to support the internationally recognised government in 2016.

"Lenderking has been doing the rounds in the region, engaging with parties to the conflict," a Gulf-based Western official told AFP.

"American involvement is bringing new momentum" to end the stalemate, the official said. "The support for Griffiths has never been stronger."

- 'Plan before the Huthis' -

Despite the optimism, Lenderking received a cool response to his proposal to kick off a revived peace process, and a source close to the UN efforts said the initiative is effectively on hold until the battle raging outside the city of Marib is won or lost.

The Huthis are throwing everything they have at the fight for the capital of an oil-rich region, sustaining heavy casualties as a price worth paying for the last piece of the north that the government still controls.

Its capture would hand the rebels an important new revenue source as well as a stronger position at the negotiating table, or even embolden them to push for more territory.

The battle is "holding back the negotiations ... because the Huthis want to see how far they can go," the source familiar with the UN efforts told AFP.

Lenderking has said he will return to the region when the Huthis "are prepared to talk".

"We now have a sound plan for a nationwide ceasefire, with elements that would immediately address Yemen's dire humanitarian situation directly," he told an Atlantic Council virtual conference on Friday after a 17-day trip to the Gulf.

"That plan has been before the Huthi leadership for a number of days," he said.

"Tragically, and somewhat confusingly for me, it appears that the Huthis are prioritising a military campaign to take Marib over ... suspending the war and moving relief to the Yemeni people."

The Huthis' chief negotiator Mohammed Abdusalam, who a source said met with Lenderking in Oman -- a popular neutral ground for negotiations -- dismissed the envoy's proposal on Friday as containing "nothing new".

However, the journalist who interviewed him on rebel-controlled Al-Masirah television later tweeted a "clarification", saying Abdusalam was commenting on the plan "in its current rather than final form, and confirmed that the debate about it is still ongoing".

- 'Very hopeful period' -

Observers say the US intervention provides a rare opportunity for a political solution, which is the only way out of the catastrophe that has befallen Yemen, the poorest country in the Arab world.

The situation is infinitely more complex than when the Huthis drove president Abdrabbuh Mansur Hadi's government out of Sanaa.

Since then, the country has fragmented, with the government battling southern separatists in a conflict dubbed a "civil war within a civil war", along with the rise of militant groups and the shifting role of Yemen's influential tribes.

Yemen's recent history is littered with failed diplomatic interventions, and the government is now extremely weak, with Hadi long exiled to Riyadh.

But the source familiar with the UN efforts described it as a "very hopeful period, despite the dramatic backdrop of what is happening in the conflict".

While it is too early to determine if progress has been made, there have been "lots of really intensified discussions and that's always a good thing".

The source said the goal is for a ceasefire to be followed by measures to open up Sanaa airport and ease restrictions on the port of Hodeida -- a key conduit for desperately needed aid -- and then the swift resumption of peace talks.