NEW DELHI: Wednesday has been a bad day for the United States-led coalition in Afghanistan, as two NATO soldiers were killed and in a separate incident, the Taliban captured the strategic district of Musa Qal.

Members of the provincial council confirmed on Wednesday that Musa Qal had falled to the Taliban following fighting that lasted all night, sending alarm bells ringing as the Taliban tighten their hold on the important Helmand province.

Also in Helmand province on the same day, in what is believed to be an insider attack, gunmen wearing Afghan military uniforms shot dead two NATO soldiers when they opened fire on a base.

NATO’s service members fired back, killing the attackers, said the organization's Resolute Support Mission in Afghanistan in a statement. The identities of the NATO soldiers who were killed have not as yet been revealed.

"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.

At the time of writing, it wasn’t entirely clear whether the attackers were members of the Afghan military or if they had procured the uniforms by other means. Initial reports, however, point toward the former, with so-called "green-on-blue" attacks -- where Afghanistan’s security forces turn their gun on foreign troops -- becoming a very real concern in the long war in Afghanistan.

The last such attack occurred in April, when an American soldier was killed in a firefight between US and Afghan troops in eastern Afghanistan. In January, three American civilian contractors were shot dead at Kabul airport by an Afghan soldier who was also killed.

The worst such attack took place in August last year, when US Major General Harold Greene was killed; at the rank of Maj Gen, Greene is the highest-ranking American service member killed by hostile action since Lieutenant General Timothy J. Maude was killed in the September 11 attacks, and the highest-ranking service member killed on foreign soil during a war since Rear Admiral Rembrandt Cecil Robinson was killed during the Vietnam War in May 1972.

Greene was killed on August 5, 2014 as he was shot in the back of the head by an Afghan soldier with an M16 rifle at Camp Qargha's Marshal Fahim National Defense University in Kabul, Afghanistan. Fourteen NATO and Afghan serviceme members were wounded in the attack, including Brigadier General Michael Bartscher of the German Bundeswehr, two Afghan generals and another Afghan officer, eight Americans, and two British soldiers.

Attacks against NATO’s presence in the country are frequent, with recent attacks including an attack a few days ago targeting a convoy of contractors with the NATO-led Resolute Support mission.

The Resolute Support Mission -- which focuses on training and support of Afghan forces -- replaced NATO's formal combat mission at the end of last year. As of the end of May, it consists of more than 13,000 troops from 40 different nations, with the United States being the single largest contributor in the form of 6000 troops.

No group has thus far claimed responsibility for the attack, although there has been a recent upswing in Taliban led violence in Afghanistan, with casualties for 2015 thus far positioning the year to be the worst yet in terms of violence. The first six months of 2015 have seen a 13 percent rise in child casualties when compared with the same period last year. The number of women killed and injured has increased by a sharp 23 percent. Total casualties have gone up too, but only by 1 percent when compared to record levels in 2014, with 1591 civilians killed and 3329 injured.

Meanwhile, the mission is readying plans to take back Musa Qala district. An unnamed official quoted in the New York Times said that “foreign forces” would be providing air support.

For now, however, these foreign forces are under threat not just from the Taliban, but Afghan security forces as well.