KABUL, Afghanistan — Two NATO soldiers were killed on Wednesday when men wearing uniforms of the Afghan security forces opened fire at an Afghan military base in the southern province of Helmand, the American-led international coalition said in a statement.

In another development highlighting the province’s precarious security, the strategic district of Musa Qala fell to the Taliban on Wednesday after heavy fighting that lasted all night, members of the provincial council said. Officials had repeatedly warned that if Musa Qala fell, the Taliban could tighten their grip on northern Helmand.

Details of the attack on the NATO soldiers were conflicting at best. The nationalities of the soldiers were not immediately clear. The statement released by the American-led coalition said the service members died when two men wearing Afghan uniforms opened fire on their vehicle. The attackers were killed when coalition forces returned fire, the statement said.

However, Afghan officials in Helmand said that the two NATO soldiers had been part of a group of Americans who had arrived at Camp Bastion earlier in the week. The officials said that the soldiers had been fired on by Afghan troops who suspected them of being Taliban fighters posing in army uniforms, a tactic the Taliban have been using in recent months.

“The guards on duty asked them for identification, but the American special forces seem to have ignored their warning,” said the head of the provincial council’s security committee, Bashir Ahmad Shakir. “The aftermath was an exchange of fire.”

A representative for the international coalition in Afghanistan declined to comment on the details of the episode.

The attack is the second time this year that members of the coalition forces have been killed by their Afghan allies, or by gunmen wearing their uniforms. The trend of insider attacks, which reached a worrying peak in 2012, has declined in recent years as the international forces in Afghanistan, which are down to about 13,000, have drastically limited their movement in the country since shifting to a mission focused on advising and training.

Despite their limited new role, the coalition forces remain heavily involved in the planning of operations and in providing air support for Afghan forces.

A senior official in the Afghan Army’s 215 Maiwand Corps, which is responsible for southern Afghanistan, said coalition officials were present on Wednesday as plans were being drawn up for an operation to take back Musa Qala district, which is a gateway to the rest of Helmand Province’s northern districts.

The official, speaking on the condition of anonymity because he was not authorized to speak to the news media, said that “foreign forces” would be providing air support in an operation beginning soon.

Musa Qala’s governor, Hajji Mohammad Sharif, said that Taliban fighters swarmed the district compound on Tuesday night, seizing government buildings, including a police station. The governor said he and other officials had hidden from the Taliban in civilian homes.

Senior Afghan officials, including the deputy minister of defense, Gen. Abdul Khaliq, have rushed to the province to try repelling an offensive by the resurgent Taliban, who are on the verge of overrunning several of its northern districts.