KABUL, Afghanistan -- Two men wearing Afghan security force uniforms opened fire Wednesday inside a military base in southern Afghanistan, killing two NATO service members before being shot dead themselves, the international force said.

NATO offered few details about the shooting in Afghanistan's southern Helmand province, which appeared to be the latest so-called "insider attack" to target foreign troops or contractors in the country. Afghan officials said they had no immediate details about the attack.

In a statement, NATO said the two men in Afghan uniforms opened fire on a vehicle with international troops inside it. Both shooters were killed when NATO forces returned fire, it said.

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NATO did not elaborate, nor did it identify the nationalities of the international troops killed nor the base the attack took place. It said the attackers wore "Afghan National Defense and Security Forces uniforms," which include the country's police, military and border patrol.

The motive for the attack was not immediately known and no group claimed responsibility for the assault. In past attacks, Taliban insurgents have been known to wear Afghan police or military uniforms to stage attacks on the international troops. Others have opened fire apparently on the own accord, like an Afghan soldier who last year killed Maj. Gen. Harold J. Greene, the highest-ranked U.S. officer to be slain in combat since 1970 in the Vietnam War.

The shooting is the third "insider attack" on foreign forces this year. In January, three American civilian contractors were shot dead at Kabul airport by an Afghan soldier who was also killed. In April, an American soldier was killed by an Afghan soldier inside the compound of the governor of eastern Nangarhar province's city of Jalalabad.

Meanwhile Wednesday, Afghan forces were at risk of being overrun after hundreds of insurgents launched a mass attack days earlier on a district headquarters in Helmand province's Musa Qala district, said Karim Atal, the head of Helmand's provincial council. Atal said the central government had yet to send reinforcements.