WASHINGTON—The international aid group Doctors Without Borders is reopening a trauma hospital in northern Afghanistan, six years after a U.S. airstrike destroyed the building and killed more than 40 people, the organization said.

The hospital has been rebuilt at a new location in the city of Kunduz, which has come under heavy attack from the Taliban in recent weeks. The hospital is expected to start operating in the coming days, close to the front lines.

The hospital was destroyed in 2015 in an attack by an AC-130 gunship during a battle in the city center between Taliban insurgents and U.S. Special Forces and Afghan commandos. The 42 people killed in the strike included 14 staff and four caretakers.

Patients burned to death in their beds, while the warplane overhead fired on survivors fleeing the wreckage. Surviving doctors and nurses tried to save their colleagues with emergency surgery in the kitchen, without anesthetic.

An employee of Doctors Without Borders walked inside the charred remains of their hospital after it was hit by a U.S. airstrike in Kunduz in 2015.

Photo: Najim Rahim/Associated Press

After initially denying it had deliberately struck the hospital, the U.S. military ultimately acknowledged that the strike had been called in by a U.S. Special Forces team on the ground who believed they were targeting a Taliban command and control center.

Doctors Without Borders, known also by its formal French name Médecins Sans Frontières, called for an independent investigation following the airstrike, but the U.S. military never agreed to one. The U.S. military later released a heavily redacted report that blamed the strike on human and technical error that led to the hospital being mistakenly identified as an enemy base.

“We still have unanswered questions about what happened that night in Kunduz,” said Chris Stokes, a senior adviser at Doctors Without Borders.

The Pentagon concluded the service members hadn’t committed war crimes because they struck the hospital unintentionally, though it disciplined 16 people for the errors that enabled the attack. President Barack Obama later apologized for the incident.

Doctors Without Borders began working to reopen the hospital soon after the attack. It took two years to secure agreements with the U.S., the Afghan government and the Taliban to respect the hospital’s neutrality. Later, construction was hindered by fighting and the Covid-19 pandemic.

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Doctors Without Borders treats all victims of war, regardless of their affiliation, barring weapons, radios and uniforms from its facilities. Afghan soldiers, police and Taliban are sometimes treated in the same wards.

Doctors Without Borders has assembled a team of about 10 international staff to prepare for the opening, and expects around 300 Afghans to work there once the center is fully operational.

The aid group is already running a temporary trauma unit at its residential compound in Kunduz, with 25 inpatient beds. The new hospital will start out with a capacity of 51 beds, with a potential to add more later on.

The U.S. offered money to help rebuild the hospital, but the organization’s policy prohibits it from taking money from governments. Families who lost relatives in the strike were given compensation.

Doctors Without Borders staff in Geneva held lights during a commemoration for the victims of the U.S. attack in 2016.

Photo: salvatore di nolfi/European Pressphoto Agency

Staff members that survived the attack have returned to Kunduz to help reopen the project. A Filipina surgeon who survived the bombing is joining the team for a nine-month tour. The international staff come from all over the world, from Brazil to Germany to Japan.

One Afghan staff member who survived the 2015 bombing said the organization and its staff feared the hospital could again be targeted. “These kinds of worries are always there,” the staff member said, “especially when you see the fighting is escalating.”

But the organization and its medical workers say the need for their services is as acute as ever.

In recent months, as the U.S. military has withdrawn almost all of its forces, the Taliban have ramped up attacks. The militant group has surrounded major cities, including Kunduz, aiming to cut off roads and supply lines. The Taliban have come close to capturing several other provinces already.

In recent weeks, the intensifying battles for major population centers have pulled the U.S. military back into battle in places like Helmand and Kunduz. U.S. airstrikes have helped beat back the insurgents and prevented the loss of any major city. But it isn’t known whether the U.S. will continue to provide air support to Afghan forces after the withdrawal is complete. President Biden has said all U.S. troops must be gone by Sept. 11.

Afghan security forces stood guard on a Humvee along a roadside in Kunduz in June.

Photo: str/Agence France-Presse/Getty Images

The 2015 bombing took place at a similarly pivotal time in the war. Mr. Obama had withdrawn most U.S. troops and said troops were only to be involved in a training mission. Instead, months later, the Taliban seized Kunduz, capturing their first provincial capital and delivering a humiliating defeat to the Afghan government after local police and soldiers fled.

The U.S. stepped in to help Afghan commandos recapture the city, and dispatched U.S. Special Forces to Kunduz. Four days later, the hospital was bombed.

The true toll of the disaster remains unknown. The hospital was the only international-run trauma center delivering lifesaving treatment to victims of war across northeast Afghanistan.

More than 68,000 emergency patients were treated there between 2011-2015. Patients were sometimes carried by family members from remote villages for days to seek medical care at the facility.

Some U.S. servicemen have always privately maintained that senior U.S. officials suppressed part of the story. They say the U.S. military had been watching the hospital and believed that Taliban fighters were holed up there.

“Truth be told, immediately after the event, I held animosity towards MSF,” said Bill Swank, one of the 16 disciplined people who was aboard the gunship and believes the Taliban were using the staff and patients at the hospital as human shields. “What happened six years ago was indeed a tragedy, and it is something that will stay with me for the rest of my life.”

Doctors Without Borders has repeatedly denied that the Taliban were using the hospital to conduct attacks.

Write to Jessica Donati at jessica.donati@wsj.com