Yemeni pro-government forces advance toward the port area of Hodeida on Wednesday amid battle for the city controlled by Houthi rebels. (Khaled Ziad/AFP/Getty Images)

Despite U.S. calls for a cease-fire, fighting is escalating in a strategic Yemeni port city, threatening the lives of hundreds of thousands of civilians and deepening a humanitarian crisis in which millions are at risk from a looming famine.

The clashes in Hodeida, pitting a U.S.-backed regional coalition led by Saudi Arabia and the United Arab Emirates against northern Houthi rebels, have caused numerous civilian casualties over the past week, according to residents, health workers and aid agencies. The violence is also edging closer to medical facilities, threatening the safety of patients and medical workers.

The fighting has particularly intensified near Hodeida’s port, through which passes more than 70 percent of all food, fuel, medicine and other essential supplies destined for the northern part of the country, where the large majority of Yemenis live. If the port is damaged, it could be catastrophic for millions of Yemenis, aid workers said.

“Hodeida is once again trapped in violence with hundreds of thousands of Yemenis caught in the middle,” Fabrizio Carboni, a senior official with the International Committee of the Red Cross, said in a statement Thursday.

Residents said they remain confined inside their houses as battles rage outside. Houthi rebels have dug in, patrolling the streets in machine-gun-mounted pickups and deploying fighters in buildings, houses, even hospitals, prompting fears that they are using civilians as human shields.

The coalition, meanwhile, has conducted scores of airstrikes and deployed U.S.-made helicopters to target areas in and around Hodeida, including residential neighborhoods.

“We have been receiving a lot of civilian casualties,” said Mareb Almahweeti, a surgeon at a military hospital in the city, adding that many of the injured were struck by shrapnel from airstrikes.

“The Apache helicopters are bombing many areas around the city most of the day,” he added. “We also hear airstrikes most of the day. The bombing is closer than before.”

After besieging Hodeida over the summer, the coalition launched a new offensive a week ago, two days after U.S. Secretary of State Mike Pompeo called for a cease-fire and peace talks within 30 days. An array of militias, aligned with the UAE and the embattled Yemeni government, pushed forward into the city, backed by airstrikes and heavy artillery.

The coalition of Sunni Muslim countries is seeking to oust the Shiite rebels and restore Yemen’s internationally recognized government, not least because the Houthis are widely said to be backed by Iran. The United States is aiding the coalition by refueling its warplanes and through intelligence and billions of dollars in arms sales.

A senior Yemeni government official said the goal of the Hodeida offensive is to militarily weaken the Houthis and push them into negotiations. Hodeida’s port is a vital source of funds and supplies for the rebels, and losing control over the city could deliver a hard blow to their military operations.

“The goal is to take over Hodeida before the 30 days,” said Moammar al-Eryani, Yemen’s minister of information. “If Hodeida is freed, the Houthis will be forced to come and sit with us at the table.”

On Wednesday, the rebels’ top leader, Abdulmalik al-Houthi, vowed in a televised address to fight back at whatever cost and urged supporters to “move seriously to counter the aggression on all fronts.”

The rhetoric and escalating bloodshed are alarming the United Nations and Western aid agencies, which in most parts of Yemen are the sole institutions assisting the 14 million people — nearly half the population — who are on the brink of famine. More than 3 million Yemenis have fled their homes; hundreds of thousands more could soon follow if the battle for Hodeida continues, aid workers said.

“The upcoming talks cannot be an excuse to disregard the laws of war that protect the lives of the Yemeni people,” Carboni said. “Wars have rules, and parties to the conflict must respect them, even in the fiercest battles.”

“This new attack on Hodeida is brushing away the hope sparked by the recent announcement of the peace talks,” he added.

Inside Hodeida, anxiety is growing. The clashes have largely unfolded in the southern and eastern parts of the densely populated city, residents said. In the affected areas, shops are either closed early or shuttered altogether. Movement is limited.

“There is a sense of fear among people from being hit by mortars or airstrikes,” said Mazen Mujammal, a 21-year-old resident.

In the first six days of the new offensive, Doctors Without Borders treated 24 civilians for war injuries in Hodeida and 50 in the town of Mokha, the medical charity said in a statement. The aid agency Save the Children said that more than 150 people have been killed in recent days.

Soon, aid workers worry, there could be fewer places to assist the population. The fighting is increasingly edging toward hospitals and health centers. On Wednesday, Save the Children reported that clashes damaged a pharmacy at one of its health facilities, adding that shelling also hit residential areas.

“The lives of hundreds of thousands of people, roughly half of them children, are in danger,” Save the Children said in a statement. “Artillery shelling is being used heavily by all sides.”

Of particular concern is al-Thawra hospital, the biggest medical facility in the city. It is mere yards from a front line where clashes are erupting daily, aid workers said. The United Nations Children’s Fund said this week that the fighting is putting 59 children at the hospital, including 25 in the intensive-care unit, “at imminent risk of death.”

“Medical staff and patients in the hospital have confirmed hearing heavy bombing and gunfire,” UNICEF said in a statement. “Access to and from the hospital, the only functioning one in the area, is now imperiled.”

More children could be endangered. Hodeida and neighboring provinces are at the epicenter of the hunger crisis, accounting for 40 percent of the 400,000 children who suffer from severe malnutrition in Yemen, UNICEF said. The sickest are often taken to al-Thawra.

At Hodeida’s 22nd May Hospital, there have been interruptions in medical services because of fierce fighting, the ICRC said.

Now, there’s a fresh concern.

Last week, Houthi rebels arrived at the hospital and took up positions on its roof, making it a potential target of the Saudi-led coalition, said Amnesty International, the watchdog group. Scores of hospitals and health clinics have been destroyed or damaged by airstrikes or shelling since the war in Yemen began more than three years ago.

“The hospital is full of injured civilians who have nowhere else to go for lifesaving medical care,” said Samah Hadid, Amnesty’s Middle East director of campaigns. “Anyone attacking a hospital under these conditions risks responsibility for war crimes.”

“The laws of war demand that hospitals are not used for military purposes,” she added.

More than half of Hodeida’s 600,000 residents have fled the city, mostly in the summer before the initial offensive. But many of those who remain are finding it difficult to leave.

According to Save the Children, temporary roadblocks erected by the warring parties were “preventing people from leaving or entering the city overnight, in effect trapping them in an active conflict zone.” Clashes on shifting front lines also have blocked escape routes to the south of the city, while the rebels have planted mines along other paths out of the city, Amnesty said.

That leaves only one northern road out. But with fuel prices rising, and the Yemeni currency collapsing because of the war, many residents don’t have the means to escape. Among them are those who returned last month during a lull in the fighting, residents said.

“Those who have come back cannot afford to leave again because they have exhausted their financial reserves,” said Nabil Almahdi, 44, a resident.

The civilians in Hodeida are “completely powerless and can only stay put to await their fate,” Hadid said. “Their lives are in the hands of warring parties who have shown little or no regard for their duty to protect civilians.”

Lise Grande, the top U.N. humanitarian official in Yemen, said: “The most vulnerable people in the whole country are sitting there in Hodeida. These are the poorest of the poor. And they can’t get out.”

If the offensive continues, she added, many of the sickest could die not only of hunger but also disease.

“Cholera remains probably the biggest threat to people in Hodeida,” Grande said. “The humanitarian programs that are operational there are making the difference in keeping them alive.”

Raghavan reported from Cairo.