Gunmen wearing Afghan military uniforms shot dead two NATO soldiers on a base in the country's south on Wednesday, the coalition said, in the latest insider attack on foreign troops.

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.

Wednesday's attack in the volatile province of Helmand is the first such incident since April, highlighting long-simmering tensions between Afghan and foreign forces.

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

"Resolute Support service members returned fire and killed the shooters," it added, without revealing the nationalities of the soldiers.

The Taliban did not immediately claim responsibility for the attack. Western officials say that most such attacks stem from personal grudges and cultural misunderstandings rather than insurgent plots.

The killings have bred fierce mistrust among local and foreign forces even as the rate of such incidents has dropped in recent years.

NATO ended its combat mission in Afghanistan in December last year, pulling out a 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.

And in April, an American soldier was killed in a firefight between US and Afghan troops in eastern Afghanistan, the first apparent insider attack since Washington announced a delay in troop withdrawals from the country.

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

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 attack" killings, 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 US-led NATO forces.

The Taliban are stepping up their summer offensive, launched in late April, amid a bitter leadership dispute following the announcement of the death of longtime leader Mullah Omar.

Mullah Akhtar Mansour, Omar's longtime trusted deputy, was named as the new Taliban chief in late July in an acrimonious power transition.
Source: AFP