DUBAI (Reuters) - The battle for Yemen's gas-rich Marib region is complicating U.S. efforts to reach a ceasefire needed to end a six-year-old war and secure a foreign policy win for President Joe Biden, two sources familiar with the talks and a diplomat said.
A U.N./U.S. peace initiative presented by Saudi Arabia in March proposed a nationwide ceasefire and the reopening of air and sea links, to bolster efforts to end a devastating conflict widely seen as a proxy war between Saudi Arabia and Iran.
Riyadh, which leads a military coalition battling Yemen's Houthi movement, has been under increasing pressure to end the war since Biden signalled Washington would no longer support the intervention and as the United Nations warns of looming famine.
But the initiative has been stuck since Yemen's Iran-aligned Houthis made a series of counter-proposals, including for a phased truce that could allow them sufficient time to seize Marib, the Saudi-backed government's last northern stronghold.
Potentially crippling the peace initiative, fighting has intensified in recent days as the Houthis push their offensive to take Marib, which if successful would strengthen the movement's hand in any future political negotiations.
"Probably the Houthis, given a choice between a ceasefire and taking Marib, would choose to take Marib," said a senior diplomat based in the region.
The peace initiative can only be saved by a "mutually hurting stalemate" in which Houthi losses reach a point where they lose tribal support, the diplomat said, adding the group has replaced seasoned fighters lost to coalition bombs with inexperienced youth.
U.S. envoy Tim Lenderking and U.N. envoy Martin Griffiths have been touring the region for discussions to try to break the deadlock and secure a ceasefire, but so far without success.
The U.N./U.S. initiative would reopen Sanaa airport, and allow fuel and food imports through Hodeidah port, both of which are controlled by the Houthis. But the movement said last month that these steps would not go far enough.
Two people involved in the talks told Reuters the main issue now is sequencing, since the Houthis insist on a full lifting of the blockade followed by a gradual ceasefire: a halt to Houthi attacks on Saudi Arabia and coalition airstrikes on Yemen, and then a truce with Yemen's government.
Coalition airstrikes are the only thing keeping Marib, home to major oil and gas fields, from falling, since Houthi forces, now 15 km (9 miles) west of the city, have more advanced weaponry than pro-government troops, military sources said.
Hundreds of fighters from both sides have been killed in the desert plain, but military and local sources say the Houthis have lost more in the war's most deadly clashes since 2018.
The Houthis, who seized swathes of Yemen's conventional military when they ousted the government from the capital Sanaa in late 2014, have sent thousands of fighters to the Kasara and Mushaja areas near Marib city whose terrain provides some cover, pro-government military and local sources said.
The fighting has displaced some 13,600 people in the region since February, according to the United Nations, which said four displacement camps were shut after being hit by shelling, injuring dozens and compounding overcrowding.
Marib hosts a quarter of Yemen's four million refugees.
The war has killed tens of thousands of people in Yemen, and caused what the United Nations describes as the world's largest humanitarian crisis with millions facing famine.
But Saudi Arabia has also felt the impact of the war. It has faced a barrage of Houthi drone and missile strikes, and is seeking security guaranties along its border as it tries to contain the influence of arch-rival Iran.
Saudi and Iranian officials discussed Yemen during direct talks this month aimed at easing tensions, six years after diplomatic ties were severed, sources said.
Michael Knights, an expert on Gulf military affairs with the Washington Institute for Near East Policy, said Saudi Arabia has enough reserves of U.S.-supplied precision-guided munitions to keep defending Marib, but time remained a factor.
The Houthis, who already control most big urban centres, have a window of time to press their offensive during hazy summer weather that reduces coalition air operations.
"If the Houthis take it, they're going to take it in the next three months," Knights said, adding that the group is advancing in pulses to seize ground and reinforce positions.
"The Houthis view Marib as a knockout blow. It makes them into a state with resources, a coastline, and most of the population. Whereas if you're (Saudi-allied Yemeni president) Hadi, it knocks you out of the game," he added.
(Reporting by Jonathan Landay in Washington and Aziz El Yaakoubi in Dubai, Additional reporting by Mohammed Ghobari in Aden; Writing by Ghaida Ghantous, Editing by William Maclean)