Defense Secretary Ash Carter addressed the attack in a statement.
"It is with deep regret that I learned today that six U.S. service members died in Afghanistan Monday," said Carter. "We are still learning all of the details, but two other service members and a U.S. contractor were also injured. They died after a vehicle-borne improvised explosive device attack on their patrol outside Bagram Air Base. It serves as a painful reminder of the dangers our troops face every day in Afghanistan."
U.S. officials said the bombing targeted a joint U.S. and Afghan patrol outside Bagram Air Base, which is the largest U.S. military base in Afghanistan. The official said the patrol was meeting with a local Afghan leader at the time of the attack.
"As I saw firsthand during my visit to Afghanistan last Friday, our troops are working diligently alongside our Afghan partners to build a brighter future for the Afghan people. Their dedicated efforts will continue despite this tragic event," said Carter. "Our deepest sympathies go out to the families of these brave Americans who died in service to this vital mission, and our thoughts remain with all of our troops serving overseas during this holiday season so that we may have peace and security at home."
There are about 9,800 U.S. troops in Afghanistan serving in a NATO mission known as "Resolute Support" to train, advise and assist Afghan military forces, with about 5,500 of them are stationed at Bagram.
Earlier, Waheed Sediqqi, a spokesman for the governor of Parwan Province, where Bagram is located, told ABC News, “This afternoon, at 1:30 p.m. local time, a convoy of Afghan and international forces were patrolling in Bagram district of Parwan province, while a suicide attacker, riding a motorbike, exploded himself.”
Sediqqi indicated that both American and Afghan troops had been killed and injured in the attack.
Twenty Americans have died in Afghanistan this year.
The U.S. combat mission in Afghanistan ended on Dec. 28, 2014. At that point the remaining 10,000 troops in Afghanistan transitioned to a training, advisory and assist mission. Most of those troops never leave the few large bases in Afghanistan where they conduct their training or advisory missions.
President Obama agreed in March to a request from U.S. military commanders to delay a scheduled U.S. troop withdrawal plan that would have reduced to 5,500 the number of troops still in Afghanistan by the end of this year.
Instead, the troop reduction was delayed until next year, and any further troop reductions would be left for the next president.
A Pentagon report last week found that the security situation in Afghanistan had deteriorated over the past six months. “The overall security situation in Afghanistan deteriorated with an increase in effective insurgent attacks and higher ANDSF and Taliban casualties,” report stated.
Though the performance of Afghan security forces has been “uneven and mixed,” the report said, the Afghan government remains in control of all major population centers “and continues to deny the Taliban strategic ground throughout the country.”