An Afghan National Police armored vehicle patrols on a street in Lashkar Gah in Helmand province, Afghanistan. (Reuters)

Two American service members were killed Wednesday in an apparent “insider” attack by an Afghan soldier at a military base in Afghanistan’s southwestern Helmand province, U.S. and Afghan officials reported.

In another development in Helmand, Taliban insurgents captured the province’s Musa Qala district after a clash with Afghan government forces, local officials said. They said 25 members of the security forces were killed in the fighting in the northern part of the province.

The attack on the foreign troops, members of a U.S.-led coalition in Afghanistan, was carried out by one person in an Afghan army uniform who opened fire on a vehicle on an Afghan base in Helmand, the coalition said in a statement.

U.S. officials in Afghanistan did not immediately disclose the nationalities of the two coalition members killed in the attack. But a defense official in the United States confirmed that both were Americans.

Other service members returned fire, wounding the assailant and another individual wearing an Afghan army uniform, the coalition statement said. Initial reports had said erroneously that two people carried out the attack and that both were killed.

Musa Qala district, Helmund province, Afghanistan.

Mohammad Karim Attal, the Helmand provincial chief, said he believed that both slain troops were Americans who were training Afghan forces at Camp Bastion, a sprawling military base in central Helmand.

Army Col. Brian Tribus, a U.S. military spokesman in Afghanistan, said that no coalition service members are “permanently located in Helmand province” at present but that “expeditionary advising teams” travel to different Afghan military bases in Helmand “to conduct their TAA [train, advise and assist] mission.”

Tribus and other U.S. officials declined to say whether the slain service members were Special Operations troops or specify on which base the shooting occurred.

In October, U.S. Marines and British forces withdrew from Camp Bastion and handed it over to the Afghan National Army.

Most remaining American forces in southern Afghanistan have been based at Kandahar Airfield in neighboring Kandahar province.

Afghan forces in Helmand have been struggling to repel repeated attacks by insurgents from the Afghan Taliban.

Attal said the confrontation Wednesday may have been sparked by a verbal dispute between coalition forces and soldiers from the Afghan National Army. But coalition troops have long been vulnerable to unprovoked attacks from rogue Afghan soldiers or those who sympathize with the Taliban.

In addition to Wednesday’s incident, there have been two other insider or “green on blue” attacks in Afghanistan this year.

In January, an Afghan soldier killed three American civilian contractors at a base near Kabul’s international airport. An Afghan soldier also shot and killed an American soldier in April in eastern Nangahar province, the Associated Press reported.

Last August, U.S Army Maj. Gen. Harold J. Greene was killed when an Afghan soldier opened fire at a military academy near Kabul, becoming the highest-ranking U.S. officer to be killed in a combat zone since the Vietnam War.

The Washington Post published a story last week detailing how Greene’s widow and other family members are questioning a Pentagon investigation into the incident.

Wednesday’s attack came amid a spike in lethal attacks on coalition troops and contractors serving in Afghanistan.

On Saturday, three civilian U.S. contractors were killed when a vehicle packed with explosives rammed a coalition convoy in central Kabul.

The three contractors worked for McLean, Va.-based DynCorp International, which provides logistical and training support to coalition and Afghan forces in Afghanistan.

In a statement, the company identified the victims of Saturday’s attack as Corey J. Dodge, 40, of Dexter, Maine; Richard P. McEvoy, 57, of Peachtree City, Ga.; and Barry D. Sutton, 46, of Calhoun, Ga.

McEvoy was a retired Army colonel, the company said.

Wednesday’s attack coincided with the seizure of the Musa Qala district by the Taliban after days of fighting.

With the fall of the strategic district, the radical Islamist insurgents can threaten Helmand’s provincial capital, Lashkar Gah, and several other districts near adjacent provinces, said Zainullah Stanekzai, a local journalist.

Dan Lamothe in Washington and Tim Craig in Islamabad, Pakistan, contributed to this report.

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