A man wearing an Afghan military uniform shot dead two American soldiers in the country's south Wednesday, the coalition said, the first insider attack on foreign troops since an embittered succession to the Taliban leadership.

No group has so far claimed responsibility for Wednesday's attack in Helmand, which coincided with the fall of a strategic district in the volatile opium-rich province to Taliban insurgents.

"Two Resolute Support (NATO) service members died early this morning when one individual wearing an Afghan (military) uniform opened fire on their vehicle at an (Afghan security forces) compound in Helmand province," a NATO statement said.

"Resolute Support service members returned fire, wounding the shooter as well as another individual wearing (military) uniform," it added in the revised statement, without revealing the nationalities of the foreign soldiers.

NATO in an earlier statement said there were two attackers and both had been killed.

A spokesman for the Pentagon said later on Wednesday "two US service members operating in support of Resolute Support mission were killed (...) when two individuals wearing Afghan national defense and security force uniforms opened fire on their vehicle at an Afghan security force compound in Helmand province".

So-called "green-on-blue" attacks -- when Afghan soldiers or police turn their guns on international troops -- have been a major problem during NATO's long years fighting alongside Afghan forces.

In another development in Helmand, Taliban insurgents overran the key district of Musa Qala after fierce fighting, according to a provincial council member.

Officials fear that the fall of the district, once a key NATO position, could help militants topple adjoining districts, tightening their grip on northern Helmand.

"Many people have been killed or injured in the fighting. We barely made it out of the district alive," said Mullah Abdul Jalil, a resident of Musa Qala who fled with his family to the provincial capital Lashkar Gah.

Taliban insurgents are ramping up their summer offensive launched in late April amid a simmering leadership succession dispute.

Mullah Akhtar Mansour was named as the insurgents' new chief in late July, but the power transition has been acrimonious.

- Surge in attacks -

Wednesday's insider attack was the first such incident since Mansour's ascension following the announcement of the death of longtime leader Mullah Omar.

The NATO statement did not give the precise location of the attack, which highlights long-simmering tensions between Afghan and foreign forces.

Western officials say most such incidents stem from personal grudges and cultural misunderstandings rather than insurgent plots.

The killings have bred fierce mistrust between local and foreign forces even though their number has declined in recent years.

The last insider attack was in April, when an American soldier was killed in a firefight between US and Afghan troops in eastern Afghanistan.

US Defense Secretary Ashton Carter at the time said the soldier's killing showed that work remains to shore up Afghan forces in the "dangerous" country.

NATO ended its combat mission in Afghanistan last December and pulled out the bulk of its troops although a 13,000-strong residual force remains for training and counter-terrorism operations.

One of the worst insider attacks took place last August when US Major General Harold Greene was killed -- the most senior American military officer to die in action overseas since the Vietnam War.

NATO troops have adopted special security measures in recent years to try to counter the threat.

The Afghan military, which has been built from scratch since the fall of the Taliban regime in 2001, has also struggled with insider attacks, high casualty rates and mass desertions.

Stretched on multiple fronts as the insurgency expands, Afghan forces are facing their first fighting season without the full support of NATO forces.