Two gunmen in Afghan defense force uniforms killed two NATO soldiers Wednesday after opening fire at a military base in southern Afghanistan, NATO officials said. The gunmen were shot dead in a counterattack from soldiers at the base.

The incident occurred in the volatile Helmand province and the motive of the attack remains clear, the Associated Press reported. NATO did not reveal the nationalities of the deceased soldiers.

No group has claimed responsibility for the attack, which is being labeled by media as the so-called “insider attack” to target foreign troops. Taliban militants have been known to don Afghan police or military uniforms to carry out shootings on international troops.

Wednesday’s attack also fuels the ongoing tensions between Afghan soldiers and NATO forces. Last August, an Afghan soldier killed Maj. Gen. Harold J. Greene, the highest-ranked officer in the U.S. Army, in the country’s capital Kabul.

On Saturday, three American contractors working with NATO were among at least a dozen people killed in a suicide bombing targeting a NATO convoy in Kabul.

NATO pulled out majority of its forces from the troubled country last December and has been working with a reduced force of about 13,000 troops to train Afghan forces combat the insurgents. Following this, the insurgents have stepped up their attacks in the country.