KABUL - Two Afghan security personnel opened fire on Western colleagues Friday, killing two US soldiers and causing a number of other casualties in two separate attacks, the military said.

US concern is mounting of the unprecedented number of such “green-on-blue” attacks, which have now killed 39 international troops in 29 such incidents so far this year, according to Nato figures.

Friday’s attacks will further erode trust between foreign troops and the Afghans they work with, a week after six American troops were killed in a single day by their local colleagues. The two Americans were killed in western Farah province, Nato’s US-led International Security Assistance Force (ISAF) said. “A member of the Afghan Local Police turned his weapon against two USFOR-A service members. The attacker was shot and killed.”

Just hours later, ISAF confirmed that “a number” of foreign and Afghan soldiers were shot and wounded by an Afghan soldier in the southern province of Kandahar.

“The attacker was shot and later died of his wounds in the hospital,” a spokesman said, without providing any further details. Local officials said two Americans and one Afghan soldier were wounded.

Nato says most of the incidents are motivated by cultural differences between troops and plays down the role of Taliban infiltration. “What we identified was that most of them were caused by personal grievances and stress situations,” the chief spokesman for ISAF, Brigadier General Gunter Katz, told AFP. “We are confident that the morale (among international troops) is still good and those incidents will not affect our transition process,” he said. Meanwhile, suicides among US Army soldiers more than doubled in July compared to June, the Pentagon said Thursday, the latest evidence of a worrisome trend that has vexed military leaders.

Among active-duty troops, 26 soldiers killed themselves last month, compared to 12 in June, according to an army statement. The July toll was the highest for any single month since the Army began documenting suicides by month in 2009, officials said.

The army, which has borne the brunt of more than ten years of protracted ground wars in Iraq and Afghanistan, has suffered the highest suicide rates among all the US armed services.

Commanders have struggled to stem the problem, funding a myriad of programmes and research to try to understand what is driving so many soldiers to take their lives.

“Suicide is the toughest enemy I have faced in my 37 years in the Army,” said General Lloyd Austin, the US Army’s vice chief of staff. “That said, I do believe suicide is preventable,” Austin said in a statement. “To combat it effectively will require sophisticated solutions aimed at helping individuals to build resiliency and strengthen their life coping skills.”

The army reported 116 suicides through July of this year — and if the current trend continues, the year’s toll would far surpass the total of 167 for 2011.

Although officials suspect repeated combat deployments have contributed to a rise in mental health problems and suicide, a significant number of soldiers kill themselves who have never been in combat.

Senior officers have sought to allay fears among many troops that seeking out counselling could be seen as a sign of weakness and damage their career prospects.