SANAA, Yemen — The shells started landing inside Hussein Ali Wuhaish’s refugee camp days after the Biden administration ended U.S. support for a Saudi-led coalition fighting in Yemen and took steps to lift sanctions on northern rebels.

The rebels, known as Houthis, seemingly emboldened by the American pivot, had intensified their offensive in strategic Marib province, one of the few safe harbors in the war. Now Wuhaish and his family, who fled to Marib in 2017, were once again on the run.

“I could hear the screams and the crying of my neighbors' children, as my wife and I tried to calm our own children,” said Wuhaish, 38, a doctor and father of five. “We realized that we were again not safe.”

“So as the sounds of the fighting got closer, we fled for our lives.”

The military escalation in Marib, the last northern stronghold of the internationally recognized Yemeni government, threatens to displace hundreds of thousands of Yemenis, many of whom have already fled violence multiple times. That would significantly worsen a humanitarian crisis already described by the United Nations as the world’s most severe, say U.S. and U.N. officials and aid workers.

The tensions could implode efforts by the United Nations and others to broker an end to the six-year-long war that has killed or injured tens of thousands in the Arab world’s poorest nation.

The conflict pits the Iran-aligned Shiite Muslim Houthis against the U.S.-backed coalition of Sunni Muslim nations, led by Saudi Arabia and the United Arab Emirates, seeking to restore the government to power. Today, the conflict is as much a proxy war between Saudi Arabia and Iran as it is a civil conflict.

For the past year, the rebels have steadily advanced in Marib province, roughly 100 miles northeast of Sanaa. Capturing it would remove the government’s last northern footprint and give the Houthis control over lucrative oil and natural gas reserves — a key ambition.

The province’s capture would strengthen the rebels’ position in negotiations to end the war and create a new government, analysts say.

“Marib was the success story of the war,” said Elisabeth Kendall, a Yemen scholar at Oxford University. “It was a haven in the middle of a war. It’s a place that provided hope. Symbolically, having that completely turn around is really tragic.”

If the Houthis seize the province, she said, “it does change the dynamic of the war.”

Biden administration ends U.S. support

For the Biden administration, the Marib offensive underscores the obstacles it faces to tackling a key foreign policy objective. Two weeks ago, President Biden ended U.S. backing for the coalition’s offensive operations, appointed a new Yemen envoy and pledged to accelerate diplomatic efforts to end the fighting.

The administration notified Congress that it would remove the Houthis from a U.S. list of foreign terrorist organizations, reversing a Trump administration decision after aid groups warned it would worsen Yemen’s humanitarian situation. Millions are on the edge of famine or suffering from diseases including the coronavirus.

The State Department warned last week that the Houthis should not interpret its pivot on Yemen as a sign of U.S. weakness and seize more territory. It urged the rebels to stop their offensive on Marib.

“The Houthis are under the false impression that this Administration intends to let its leadership off the hook,” spokesman Ned Price said. “They are sorely mistaken.”

“The quest for territorial gain by force threatens all of the prospects of the peace process,” he said last week.

Mohammed Ali al-Houthi, a senior Houthi official, denounced Griffiths’ characterization of the offensive as an “attack.” He blamed the United States for the conflict.

“Six years have passed since Obama and Biden decided to launch the aggression and test the patience of the Yemeni people,” al-Houthi tweeted. “We can never accept a peace that makes us subdued, and if we were to accept that, we would have accepted it in the early days.”

‘We have nowhere to go now’

In the conflict’s first years, Marib province was spared heavy fighting, allowing local authorities to channel oil and gas revenue into developing the economy.

But that stability also attracted at least 850,000 people displaced from areas gripped by fighting, according to the International Organization of Migration, most crowding camps outside Marib city. The conflict’s escalation would be catastrophic for the displaced, say the United Nations and aid agencies.

“Many are in urgent need of food, water, health care and other humanitarian assistance,” said Muhsin Siddiquey, Yemen country director for Oxfam. “Many families are especially vulnerable as they have already been forced to flee multiple times and may not have the resources to flee again.”

The Houthis are pouring in heavy weaponry and equipment as they push toward the city, according to local reports and aid agencies. Scores of fighters on both sides have been killed in recent days and several displaced people’s camps have been shelled, according to aid agencies and local reports.

Wuhaish’s family fled their village in Dhamar province after Houthi fighters abducted, jailed and tortured him for speaking out against them, he said. They settled down in the provincial capital, Dhamar city, but in 2017, the Houthis came looking for him again. That’s when they fled to Al Zour camp, on the outskirts of Marib city.

This month, their lives were upended again.

As the bombs fell, they fled their camp with only the clothes on their back, Wuhaish said. He said many families are now sleeping in tents or out in the open.

Wuhaish’s family was fortunate to stay temporarily in a friend’s house. But finding their next refuge could prove difficult. They have little money. The Houthis control the areas west of Marib. To the east is a vast desert with little water. Returning to Dhamar could mean death.

“We have nowhere to go now,” said Wuhaish. “I just don't know what tomorrow will bring.”