Four Americans, including two soldiers and two contractors, were killed Saturday by a suicide bomber at the Bagram Airfield, the largest US military airbase in Afghanistan's central Parwan province, according to the US military. The attack struck just after 5:30 a.m. (0100 GMT).

US Secretary of Defense Ash Carter said he was "deeply saddened" by the news and expressed his "sincere condolences" to the families of the victims.

"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," he said.

As for the attackers, he added, "my message is simple. We will not be deterred in our mission to protect our homeland and help Afghanistan secure its own future."

Security is stepped-up around Bagram air base

Waheed Sediqi, spokesman for the governor of Parwan province, said the blast was caused by a suicide attacker who blew himself up near a dining facility. "We don't know the identity of victims yet, but the attacker was one of the Afghan laborers working there," Sediqi said.

In a statement, US Army General John W. Nicholson, the commander of NATO's "Resolute Support" mission, confirmed that four were killed and at least 14 wounded. "To the family and friends of those who lost their lives today, we share your loss and our thoughts are with you," Nicholson said in a statement. "We offer you our deepest condolences."

The Taliban's spokesman Zabihullah Mujahid took to Twitter to take responsibility and claimed a heavy death toll.

Bagram Airfield, which lies about 60 kilometers (36 miles) north of the capital, Kabul, has frequently come under attack by Taliban insurgents. A motorcycle-riding Taliban suicide attacker killed six US soldiers near the same US military base last December in one of the deadliest attacks on foreign soldiers in 2015.

Nearly 10,000 US troops are stationed in Afghanistan, assisting Afghan security forces under the Train, Advise and Assist (TAA) mission. Four people were killed and 128 others were wounded Thursday when a suicide bomber drove a truck laden with explosives into the German consulate in the northern city of Mazar-e Sharif. The Taliban said the attack was in retaliation for US air strikes near the northern city of Kunduz last week which killed more than 30 civilians.

jar/tj (Reuters, dpa, AFP)