Six American troops were killed Monday in a suicide car bombing near the largest U.S. military base in Afghanistan, officials said.
"We're deeply saddened by this loss," U.S. Army Brig. Gen. Wilson A. Shoffner, spokesman for the U.S.-led military coalition, said in a statement. "Our heartfelt sympathies go out to the families and friends of those affected in this tragic incident, especially during this holiday season."
"It serves as a painful reminder of the dangers our troops face every day in Afghanistan," Carter said. "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."
A member of the provincial council of Parwan, who spoke on condition of anonymity, said at least three members of the Afghan National Police were injured.
Bagram Airfield is home to one of the largest contingents of the remaining 9,800 U.S. troops in Afghanistan.
The bombing came amid reports that the Taliban had taken over the district of Sangin in the southern province of Helmand. The volatile district was the scene of heavy fighting between insurgents and British forces until 2010, when U.S. Marines took over responsibility for the area. The last Marines withdrew from Sangin in May 2014.
Though North Atlantic Treaty Organization military forces technically ended their combat mission last December, about 17,000 coalition troops remain in the country. The total includes the U.S. troops.
About 5,000 U.S. troops are based at Bagram Airfield, a vast military installation where F-16 fighter jets, armed drones, and cargo planes fly on daily missions. U.S. troops still accompany Afghan forces on security patrols around the base to guard against Taliban and other militant groups.
During the daylong visit to Afghanistan, Carter was briefed on the security threat throughout the country from a resurgent Taliban and a growing number of Islamic State fighters.
"We expect that they're going to keep fighting, because the enemy isn't going to go away overnight," he said in Jalalabad.
A Pentagon study released this week concluded that security in Afghanistan was worsening by almost any measure. It said the Taliban "likely will try to build momentum from their countrywide attack strategy" next year.
Pentagon records show 16 U.S. troops and one civilian have died in Afghanistan this year, and an additional 68 were wounded in action.
Later Monday, Kabul, the Afghan capital, was hit by three rockets, officials said. There were no immediate reports of casualties, but the rockets reportedly landed near embassies and government buildings.