A day after the White House declined to classify the Taliban as a terror group, the group claimed responsibility Friday for an apparent insider attack at Kabul airport in which three American contractors and an Afghan were killed.

Details of the Thursday evening shooting are still unclear, with a spokesman for NATO's Resolute Support mission saying the incident is under investigation.

"They do carry out tactics that are akin to terrorism. They do pursue terror attacks in an effort to try to advance their agenda," White House Press Secretary Josh Earnest said in a press conference on Thursday, but "they have a different classification."

World News Videos | ABC World News
World News Videos | ABC World News

Semantics aside, the Taliban is suspected in multiple attacks over just the last 48 hours that have killed more than 30 people, including a suicide bombing attack on a funeral in Afghanistan on Thursday that killed 16 and wounded 39.

A US defense official in Washington told AFP that the American victims, who were employed under a Defense Department contract to help train the Afghan air force, died from gunshot wounds.
Taliban spokesman Zabihullah Mujahid said a member of the insurgent movement was responsible for the attack.

"A brave Afghan mujahid infiltrator working in the military side of Kabul airport opened fire on invading American soldiers, killing three Americans," he said in a statement sent to media.

Western troops and civilians training Afghan security forces have faced lethal assaults from Afghans in uniform who turn their guns on their counterparts.

NATO troops have adopted special security measures in recent years to try to counter the threat.

The airport in the Afghan capital is heavily guarded, with one section devoted to commercial aircraft and another area set aside for a NATO contingent.

Thursday's attack came after a surge in Taliban violence over the past year.

The funeral was for four people, including a police commander, killed earlier Thursday in a roadside bomb attack.

Most NATO combat troops pulled out of Afghanistan last year but a small contingent of about 12,000 remain in the country, including roughly 10,600 American forces.

The American soldiers, along with other NATO troops and private contractors, are helping the Afghans improve their logistics and build up a fledgling air force.