Two men wearing Afghan National Defense and Security Force uniforms opened fire Wednesday inside a military base in southern Afghanistan, killing two US NATO service members before being shot dead themselves, the international force said.

At the Pentagon Wednesday, Captain Jeff Davis told reporters he would not release their names or service citing notifications of next of kin and a 24 hour waiting period.

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 men opened fire on a vehicle with international troops inside it. The forces includes military and border patrol. Both shooters were killed when NATO forces returned fire. 

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, Taliban insurgents overran the Musa Qala district in Helmand province, said Mohammad Sharif Musa, the district's administrative chief. He said Afghan security forces had been resisting the Taliban attack for more than a week.

"We tried hard in the past days to fight the Taliban, but we didn't get any support from the government and finally we lost control of the district," Sharif said.

He said there were casualties among the Afghan security forces, though he couldn't provide casualty figures.