Six American troops were killed on Monday when a suicide bomber attacked their patrol near Bagram air base in Afghanistan, a US official told Reuters, speaking on the condition of anonymity.
NATO's Resolute Support mission had earlier provided the number of service members killed but did not offer their nationality.
The Taliban claimed responsibility for a suicide attack.
Brigadier General William Shoffner, the spokesperson for the NATO-led Resolute Support mission, told reporters that the attack took place around 1:30pm local time.
"We're deeply saddened by this loss," he said. "Our heartfelt sympathies go out to the families and friends of those affected in this tragic incident, especially during this holiday season."
At the time, Shoffner would not elaborate on the nationalities of the deceased, but Bagram airbase is the largest US military facility in the country.
According to Waheed Sediqi, spokesperson for the governor of Parwan province, the soldiers were on patrol close to Bagram airbase when they were attack by a suicide attacker riding a motorcycle. He confirmed that three Afghan police officers were injured in the same incident.
Taliban spokesperson Zabiullah Mujahed declared responsibility for the attack on Twitter, claiming that 19 people had been killed or wounded, though there was no way to corroborate those numbers.
Bagram is located 25 miles to the north of Kabul and is a major base for the nearly 9,800 US troops who are still operating in Afghanistan. The base is often the target of Taliban attacks. A Georgian soldier was killed an assault on the facility last September, and a US civilian was killed there this past June in what the US military called an "indirect fire" incident.
The attack on Monday was the deadliest Taliban operation since last summer, and is part of a growing trend of the insurgency's increasingly brazen attacks in the country. Last week, the group launched a deadly raid on the Kandahar airbase in the south and a separate attack on the Spanish Embassy guesthouse in Kabul.
The Taliban is gaining ground in southern Afghanistan as well. On Sunday, the deputy governor of Helmand province made a public plea to Afghan President Ashraf Ghani for support, warning that his province was in danger of falling to the group after months of heavy fighting. At least 90 members of the security forces have been killed in the past few days in a pitched battle with the Taliban.
In an unrelated attack on Monday, Lisa Akbari, an Afghan-American aid worker, was killed in her apartment building. The US Embassy confirmed the attack, but declined to offer more details about who was responsible. Afghan officials confirmed that a suspect had been taken into custody, but did not provide details about a potential motivation or affiliation.
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