Two U.S. service members and two American contractors were killed in a suicide bomb attack in Afghanistan, an official said.
The incident occurred shortly after 5:30 a.m. local time Saturday morning at Bagram Airfield, the largest air base in Afghanistan and located north of Kabul, according to news reports.
In addition to causing fatalities, the explosion wounded 16 other U.S. troops and one Polish soldier, Defense Secretary Ashton Carter said.
"I am deeply saddened to learn that an explosion early this morning at Bagram Airfield in Afghanistan has resulted in U.S. casualties," he said in a statement. "I want to express my sincere condolences to the families of the fallen, and I want to reassure the loved ones of those injured that they are getting the best possible care."
Carter added, "Force protection is always a top priority for us in Afghanistan, and we will investigate this tragedy to determine any steps we can take to improve it."
Some 14,000 troops, contractors and civilians work at the facility, which was placed under lockdown and heightened security after the incident.
The attacker entered with a group of workers headed to work at the base early in the morning, according to Abdul Wahid Sediqi, spokesman for the governor of Parwan Province, where the installation is located, The New York Times reported.
"For those who carried out this attack, my message is simple," Carter said. "We will not be deterred in our mission to protect our homeland and help Afghanistan secure its own future."
The Taliban claimed responsibility for the attack, which reportedly targeted a post-Veterans Day run at the installation, according to news reports.
Taliban spokesman Zabihullah Mujahid said a suicide bomber, who died in the attack, targeted "a sports ground where more than 100 military officers, important people and soldiers were exercising," according to a statement, CNN reported.
The attack brings the total number of U.S. troops killed in Afghanistan this year to 13 and the total number of coalition forces killed to 15, according to statistics compiled by the website icasualties.org.
Since the U.S. military launched military operations in Afghanistan following the Sept. 11, 2001, terrorist attacks in New York and Washington, D.C., a total of 3,527 coalition members have died in the country, including 2,391 American troops, according to the website.
The U.S. plans to slightly decrease the number of troops in Afghanistan to about 8,400. Many American service members are stationed at Bagram, which has previously been targeted by Taliban fighters.
Last December, a Taliban suicide bomber riding a motorcycle killed six U.S. soldiers near the base in one of the deadliest attacks on foreign troops in the country in 2015, Agence France Presse reported. Saturday's assault came after a Taliban truck bomb exploded at the German consulate in Afghanistan's northern Mazar-i-Sharif city on Thursday, killing at least six people and wounding more than 100 others, according to Agence France Presse.
"My thoughts go to the loved ones of those killed at #Bagram Airfield & the wounded," NATO Secretary General Jens Stoltenberg said on Twitter. "#NATO stands w/ #Afghanistan in the fight against terror."