A U.S. army infantryman walks in front of a mine-resistant vehicle at Lindsey Forward Operating Base Saturday in Kandahar.

Story highlights

The attack involves suspected Afghan police, an official says

The nationalities of the troops killed are not immediately released

On Saturday, two British troops are killed by an Afghan police officer

CNN  — 

Four NATO troops were killed Sunday in an attack believed to involve Afghan police in southern Afghanistan, officials said, in what appears to be the latest incident of local forces turning their guns on coalition partners.

The troops died “following an insider attack involving suspected Afghan police,” said Jamie Graybeal, a spokesman with NATO’s International Security Assistance Force. No more details were provided.

The attacks followed news that insurgents disguised in U.S. Army uniforms carried out a coordinated assault Friday at a joint American-British base in the same region. That assault raised concerns that the attackers had inside knowledge.

ISAF: Insurgents wore U.S. Army uniforms

It is too early to conclude whether the attackers had “inside knowledge,” Graybeal said. The incident is under investigation.

The international coalition released details that suggest it was a well-coordinated assault on the fortified base in a remote desert region in Helmand province.

Two U.S. Marines and 14 insurgents died in a firefight. The lone surviving insurgent, who was wounded, was taken into custody, the international coalition said in a statement.

The insurgents – organized into three teams – waged an assault on the airfield of the joint base that includes American-run Camp Leatherneck and British-run Camp Bastion, where British royal Prince Harry is stationed.

Six AV-8B Harrier jets were destroyed and two others were damaged, international forces said. Six aircraft hangers suffered damage, while six refueling stations were destroyed, it said.

The international coalition gave no indication how the insurgents obtained U.S. Army uniforms.

NATO denies civilians killed in air strike

The Taliban have claimed responsibility for the attack, calling it a response to the 14-minute movie trailer “Innocence of Muslims” that sparked protests in more than 20 countries. The trailer, which depicts the Prophet Mohammed as a child molester and ruthless killer, has roiled the Muslim world,

In Afghanistan, the Taliban have called on insurgents to step up attacks against the United States and its NATO allies in retaliation.

Two British troops were killed Saturday when an Afghan police officer opened fire on them at a checkpoint in Helmand province’s Nahr-e Saraj district, the British Defence Ministry said.

Meanwhile, the international coalition declined to release details about the nationality and location of the attack that killed four troops in southern Afghanistan .

Those deaths, including the two killed Saturday, raised the number to more than 50 coalition troops killed in “green-on-blue” or “insider” attacks in Afghanistan this year.

“Green-on-blue” refers to a color coding system used by the military, in which blue refers to the friendly force, in this case ISAF; and green refers to allied friendly forces, in this case Afghan National Security Forces.

CNN’s Chelsea J. Carter, Chris Lawrence and Joe Sterling contributed to this report.