John Bacon

USA TODAY

Three U.S. soldiers were wounded Sunday when an Afghan officer opened fire at a base in Helmand province, a U.S. military spokesman said.

Coalition security forces at the scene returned fire "to end the attack" and the officer was killed, Navy Capt. Bill Salvin said. The soldiers were receiving medical treatment after the shootout at Camp Antonik, Salvin said.

Afghanistan's TOLO news reported that the incident happened during a military training exercise. TOLO, citing military officials, said the attacker deliberately fired on the U.S. forces. Col. Mohammad Rasoul Zazai, an Afghan army spokesman, told the Associated Press that the soldier made a “mistake” and had not fired deliberately.

A rash of so-called "green on blue" shootings of American troops by Afghan soldiers stymied military leaders in the earlier years of the war. The incidents have been few and far between since the vast majority of foreign troops left the country in 2014. In October, however, Sgt. Douglas Riney, 26, and military contractor Michael Sauro were killed by a man wearing an Afghan army uniform, NATO officials said.

Also Sunday, at least 13 Taliban militants were killed and about a dozen others were wounded during operations in southern Zabul province, the governor’s spokesperson, Gul Islam Siyal, told Pajhwok Afghan News. Two Afghan soldiers were wounded during the operation, the AP said.

Also, two Taliban leaders were killed in an apparent drone strike in the eastern Paktika province, the AP reported. Still, Kabul's U.S.-backed government is struggling to retain control of the country from Taliban rebels. About 57% of districts nationwide are under government control, down 15% from November 2015.

Army Gen. John Nicholson, commander of the U.S.-led international military force in Afghanistan, said last month the war has essentially reached a stalemate. He said he would like to add about 1,400 U.S. troops to the 8,000-plus U.S. contingent to improve battlefield surveillance and move trained advisers deeper into Afghan forces to bolster leadership.

Nicholson did not advocate the return of American combat forces to the country, but he described more robust support for Afghanistan’s military. The Trump administration is reviewing U.S. military strategy in Afghanistan, and the White House has not said whether it would authorize an increase in U.S. forces.