Five American troops were killed yesterday during a security operation in southern Afghanistan. Investigators are looking into the likelihood that friendly fire was the cause. Our thoughts and prayers are with the families of these fallen, Pentagon spokesman Rear Admiral John Kirby said on Tuesday.

Zabul provincial police chief General Ghulam Sakhi Rooghlawanay said there was a joint operation by Afghan and foreign troops in the Arghandab district of the province on Monday.

As the mission drew to a close, the troops came under attack from Taliban militants and called in air support. Five foreign soldiers and one Afghan army officer were killed mistakenly by the US-led air strike, Rooghlawanay pointed out.

According to official figures released by the website icasualties.org, the deaths bring to 7 the number of fatalities among foreign soldiers in June.

The website added that the overall number of ISAF soldiers killed so far this year in Afghanistan stands at 40, out of which 29 are American.

A total of 160 US-led soldiers lost their lives in Afghanistan in 2013. However, 2010 remains the deadliest year for US-led military casualties, with a death toll of 711.

There are currently more than 50,000 foreign troops in Afghanistan, including some 33,000 US troops.

The increasing number of military casualties in Afghanistan has caused widespread anger in the US and other NATO member states, undermining public support for the Afghan war.

The US and its allies invaded Afghanistan in 2001 as part of the so-called war on terror. The offensive removed the Taliban from power, but the country is still gripped by insecurity.