Heavy fighting between government forces and Houthi rebels has killed more than two dozen people in the past three days in the central Marib province, Yemeni officials and tribal leaders said Tuesday.

The war in Yemen erupted in 2014, when Iran-allied Houthi rebels seized the capital and much of the country's north. A Saudi-led coalition, determined to restore the authority of President Abed Rabbo Mansour Hadi's government, launched a sweeping military intervention months later.

The officials said at least 29 people were killed and dozens were wounded from both sides in the most recent fighting for Marib. The rebels have sought to take control of the oil-rich province from the internationally recognized government.

Tribal leaders said the Houthis have deployed reinforcements to break government defenses in Marib, an anti-Houthi stronghold, but they have made no progress.

The officials spoke on condition of anonymity because they were not authorized to brief the media, as did the tribal leaders for fear of reprisals.

U.N. envoy to Yemen Martin Griffiths said he was ``extremely concerned'' about the Houthi attacks on Marib, ``especially at a time of renewed diplomatic momentum'' to resume talks aiming to end the war.

``A negotiated political settlement that meets the aspirations of the Yemeni people is the only sustainable solution to end this conflict,'' he tweeted.

Mohamed Abdel Salam, a spokesman for the Houthis, denounced Griffiths' comments. He called for the U.N. envoy to ``adjust his crooked logic.'' He did not elaborate.

The new bout of fighting in Marib came after President Joe Biden announced last week that the U.S. will end its support, including some arms sales, of the Saudi-led coalition's war against the Houthis.

The long-awaited move refocused a spotlight on the world's worst humanitarian crisis and was welcomed by many Yemenis and aid groups that hope the policy change might add to momentum for peace talks.

Griffiths also visited Iran, the main backer of the Houthis, on Saturday, part of a broader effort to negotiate a political solution to the Yemen conflict.

The war in Yemen has spawned the world's worst humanitarian crisis, leaving millions suffering from food and medical shortages. It has killed some 1130,000 people, including fighters and civilians.