repel them, adding to the insurgents' recent advances in a heavily fought over region of opium farms and trade routes.
Elsewhere in Helmand, a man in Afghan uniform opened fire in the former British base of Camp Bastion, killing two U.S. service personnel, before being shot and wounded. Another man in Afghan uniform was wounded in the return fire.
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.
Pentagon spokesman Captain Jeff Davis said the attacker opened fire on the vehicle in which the Americans were traveling. He gave no more details.
An Afghan regional official said the incident involved Afghan special forces firing on allies at the former Camp Bastion, which was handed over to Afghan forces last year.
Helmand's Musa Qala district fell after the Taliban overran police and army posts to retake a district straddling smuggling routes that was wrenched from them by British and Afghan troops eight years ago.
U.S. warplanes have been bombarding Musa Qala since the weekend, killing up to 40 militants, with two new air strikes on Tuesday. But they regrouped, chasing the district government out of town and confiscating weapons in what a spokesman for Afghanistan's 215th Maiwand Corps called a "tactical retreat" to protect civilians.
Coalition military advisors have recently been working with the 215th Corps, which based in Helmand capital Lashkar Gah.
"Afghan special forces, police and commandos have been deployed to Helmand in order to retake Musa Qala district. Foreign air strikes are backing our forces," said Dawlat Waziri, a spokesman for the defense ministry.
In Afghanistan's first summer fighting season since foreign troops stepped back from combat roles, the Taliban have pushed into several districts in the North and South but have mostly struggled to keep hold of them.
Last week, Afghan forces pulled out of the town of Nawzad, the headquarters of a neighboring district that was also fiercely fought over when British and U.S. forces were stationed in Helmand, the country's main opium production center.
That means the Taliban currently control three districts in northern Helmand and have partial control of several others, including Kajaki, where they frequently disrupt supplies from a large U.S.-built hydroelectric dam powering the province.
"We left the district early in the morning because the Taliban were attacking from all sides," Musa Qala 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.
In the years following the 2001 U.S. invasion that toppled the Taliban government, more than 400 British soldiers died in Helmand, several while defending Musa Qala. More than 350 U.S. Marines also lost their lives in the province.
Nearly 14 years later and after foreign forces formally ended their combat mission, the Taliban is still fighting a guerrilla war aimed at restoring their hardline regime.
Violence has increased sharply across Afghanistan since the coalition mostly withdrew in December, leaving a small contingent of about 12,000 NATO troops to train Afghan forces.