The number of American military personnel killed in the Afghan war has reached 2,000 following an insider attack in the country's east.
The death toll in Afghanistan has climbed steadily in recent months with a spate of attacks by Afghan army and police against American and Nato troops.
The attacks have raised questions about whether allied countries will achieve their aim of helping the Afghan government and its forces stand on their own after most foreign troops leave.
A US official confirmed the latest death on Sunday, saying an American serviceman was killed in an apparent insider attack by Afghan forces in the east of the country late on Saturday.
The official spoke on condition of anonymity because the nationality of those killed had not been formally released.
A civilian contractor with Nato and at least two Afghan soldiers also died in the attack, according to a coalition statement and Afghan provincial officials. The nationality of the civilian was not disclosed.
The death of the 2,000th American service member is a cold reminder of the human cost of an 11-year-old conflict that garners little public support in the US, as the country prepares to withdraw most of its combat forces by the end of 2014.
Attacks by Afghan soldiers or police - or insurgents disguised in their uniforms - have killed 52 American and other Nato troops so far this year.
The so-called insider attacks are considered one of the most serious threats to the US exit strategy from the country.
The invasion of Afghanistan in 2001 targeted al Qaeda and its Taliban allies following the September 11 attacks, which claimed nearly 3,000 lives in the United States.
Victory in Afghanistan seemed to come quickly. Kabul fell within weeks, and the hard-line Taliban regime was toppled with few US casualties.
But the Bush administration's shift toward war with Iraq left the Western powers without enough resources on the ground, so by 2006 the Taliban had regrouped into a serious military threat.
President Obama sent more troops to Afghanistan, where casualties have increased sharply in the last few years.
But the American public has grown weary of having its military in a perpetual state of conflict, especially after the withdrawal of American troops from Iraq at the end of last year.
That war, which began with a US-led invasion in 2003 to oust Saddam Hussein, cost the lives of nearly 4,500 US troops, more than twice the number that have died in Afghanistan so far.