Taliban bomber kills four Americans at NATO base in Afghanistan

• Trump says Clinton policy on Syria would lead to World War Three
• Syrian army and Russian jets halt Aleppo bombardment for second day
• North Korea: “United States of American's nuclear weapon monopoly were over”

Two US military service members and two US contractors were killed, and 16 other US service members were wounded, along with a Polish soldier who was part of the NATO mission, US Defense Secretary Ash Carter said in a statement.

"For those who carried out this attack, my message is simple. We will not be deterred in our mission to protect our homeland and help Afghanistan secure its own future," Carter said.

Afghan National Army soldiers keep watch outside the Bagram Airfield entrance gate, after an explosion at the NATO air base. (Photo: Reuters)

It was the deadliest attack on US soldiers in the country since a suicide bomber on a motorcycle killed six American troops on patrol near Bagram in December 2015.

Carter said the Pentagon will investigate November 12 attack to determine what steps could be taken to improve protection for the base.

The attack, which was claimed by the Taliban, underlines the foreign policy challenge that will face US President-elect Donald Trump when he takes office in January.

President Barack Obama had originally hoped to have all US forces out of the country by the end of his term, but was forced to abandon that goal as Afghan forces struggled to contain the Taliban insurgency.

Under current plans, 8,400 US troops will remain as part of the Resolute Support operation and a separate US counter terrorism mission after Obama decided to slow down a planned reduction of the force, leaving it to his successor to decide future strategy.

Waheed Sediqqi, spokesman for the Parwan provincial governor, said the bomber managed to enter the heavily protected site, the largest US base in Afghanistan, and was standing in a line with Afghan laborers when he detonated a suicide vest.

The NATO-led Resolute Support mission response teams at the airfield were treating the wounded and investigating the incident.

It follows a suicide attack on the German consulate in the northern city of Mazar-i-Sharif late on November 10 night that killed four people and wounded more than 100 others. That attack was retaliation for air strikes near the northern city of Kunduz last week which killed more than 30 civilians.

The Taliban's spokesman, Zabihullah Mujahid, said November 12 attack, which he said had been planned for four months, had caused heavy casualties, killing 23 Americans and wounding 44. The movement often exaggerates the number of casualties caused by its operations.