A man dressed in an Afghan army uniform opened fire Tuesday on foreign troops at a military base, causing casualties, an Afghan military spokesman said.

   NATO said it was investigating an "incident'' involving both Afghan and international troops at Camp Qargha, a base west of the capital, Kabul, which trains officers for the country's army. 

   Gen. Mohammmad Zahir Azimi, a spokesman for Afghanistan's Defense Ministry, wrote on Twitter that "a terrorist using (the) uniform of (the) Afghan Army'' opened fire, wounding some. He did not elaborate. Afghan officials declined to immediately comment. 

   In its statement, NATO said that it was "in the process of assessing the situation.''

   The attack comes as so-called "insider attacks'' -  incidents in which Afghan security turn on their NATO partners - largely dropped last year. In 2013, there were 16 deaths in 10 separate attacks. In 2012, such attacks killed 53 coalition troops in 38 separate attacks.

   Such "insider attacks'' are sometimes claimed by the Taliban insurgency as proof of their infiltration. Others are attributed to personal disputes or resentment by Afghans who have soured on the continued international presence in their country more than a dozen years after the fall of the Taliban's ultra-conservative Islamic regime.  

   Foreign aid workers, contractors and other civilians in Afghanistan are increasingly becoming targets of violence as the U.S.-led military coalition continues a withdrawal to be complete by the end of the year.  

   Meanwhile Tuesday, a NATO helicopter strike targeting missile-launching Taliban militants killed four civilians in western Afghanistan, an Afghan official said Tuesday. NATO said they were investigating the attack. 

   The attack in western Herat province comes as civilian casualties from NATO attacks remain a contentious issue across the country. Almost 200 people protested against NATO in Herat on Tuesday, carrying the bodies of the dead civilians into the provincial capital and demanding an investigation. 

   The strike happened Monday night in the province's Shindan district, said Raouf Ahmadi, a spokesman for the provincial chief of police. He said Taliban militants launched a missile at an airport nearby, drawing the NATO helicopter's fire. He said the NATO attack killed two men, one woman and a child. 

   In a statement, NATO said it was aware of the attack and was investigating, without elaborating.

   NATO "takes all allegations of civilian casualties seriously, and is assessing the facts surrounding this incident,'' it said. 

   Civilians increasingly find themselves under fire as the 2001 U.S.-led war draws to a close, as Afghan forces take the lead in operations targeting the Taliban. The civilian death toll in the war in Afghanistan rose 17 percent for the first half of this year, the United Nations reported in July. The U.N. said 1,564 civilians were killed from January through June, compared with 1,342 in the first six months of 2013. It blamed   

   Insurgents were responsible for 74 percent of the casualties, the U.N. said, while pro-government forces were responsible for 9 percent, government forces 8 percent and foreign troops just 1 percent. The rest could not be attributed to any group. 

   Outgoing President Hamid Karzai has repeatedly clashed with NATO over civilian casualties. 

   Afghan security forces also increasingly find themselves under attack as the planned foreign troop withdrawal draws near. On Tuesday, a police car struck a roadside bomb in the eastern province of Nouristan, killing three officers, provincial police chief Abdul Baqi Nouristani said. (AP)