Two US military personnel were shot dead at an Afghan military base.
The Taliban seized a district headquarters in Afghanistan's Helmand province on Monday despite US air strikes to repel them, and two US military personnel were shot dead at an Afghan military base in the area by men wearing Afghan uniforms.
The district of Musa Qala fell after the Taliban overran police and army posts in an offensive that lasted several days. Three US air strikes on Saturday killed up to 40 militants, but they regrouped and chased government officials out of town.
Elsewhere in Helmand, two men in military uniforms opened fire in the former British base of Camp Bastion, killing two US service personnel, before being shot dead themselves.
Pentagon spokesman Captain Jeff Davis said the attackers were wearing Afghan uniforms and opened fire on the vehicle in which the Americans were travelling. He gave no more details.
It was the second incident this year involving Afghan troops, or people wearing Afghan uniforms, shooting at foreign soldiers. No group has claimed the attack.
An Afghan regional official said the incident involved two apparent Afghan special forces soldiers. Camp Bastion is a major base that was handed over to Afghan forces last year.
In the first summer fighting season since foreign troops formally stepped back from combat roles in the Afghan war, the Taliban have pushed into a number of districts but have struggled to hold them when the Afghan army counter-attacks.
Musa Qala and neighbouring Nawzad, which recently fell to the Taliban, saw some of the most lethal battles between Taliban insurgents and British and US forces following toppling of the hardline Islamists' five-year rule in 2001.
Nearly 14 years later, the Taliban is still fighting a guerrilla war aimed at returning to power.
"We left the district early in the morning because the Taliban were attacking from all sides," district governor Mohammad Sharif told Reuters by telephone.
"We had asked for reinforcements for days, but none arrived and this was what happened," he said.
Strong through much of Helmand province, which is the largest producer of Afghanistan's lucrative opium crop, the Taliban killed more than 400 British soldiers there during a counterinsurgency campaign that ended last year.
Violence has increased sharply across Afghanistan since foreign forces mostly withdrew in December, leaving a small contingent of about 12,000 NATO troops to train Afghan forces.
© Thomson Reuters 2015