By Daniel De Luce (AFP) – 3 hours ago

BAGRAM, Afghanistan — The top US military officer Martin Dempsey flew to Afghanistan Sunday for talks with senior commanders, as this year's toll from insider attacks by Afghans on coalition troops reached 40.

In the latest incident an Afghan in police uniform shot and killed a NATO soldier in southern Afghanistan Sunday, the military said.

The spike in so-called green-on-blue attacks is causing growing concern in the United States and among its NATO allies fighting the Taliban insurgency in Afghanistan.

Before landing at the sprawling US air base in Bagram, chairman of the US Joint Chiefs of Staff Martin Dempsey told reporters the rise in the attacks would be a key focus of his meetings with US-led coalition and Afghan officers.

"As far as the insider threat, of course that's going to be a topic," Dempsey told AFP and Fox News.

Despite expanded efforts to screen recruits and preempt potential turncoat attacks, Dempsey said the number of incidents continues to increase.

"We have an eight-step vetting process that's been in place in earnest for about a year. But we haven't turned the corner on the trend," he said.

Some of the attacks are claimed by the Taliban, who say they have infiltrated the ranks of Afghan security forces, but many are attributed to cultural differences and antagonism between local and US-led allied forces.

The International Security Assistance Force (ISAF) said in a statement the latest death came when "an individual wearing an Afghan Uniformed Police uniform turned his weapon against ISAF service members", without specifying the soldier's nationality.

NATO does not disclose the number of wounded in any attacks, but a provincial official said another soldier and an interpreter were injured and that the shooter was killed by return fire.

The incident happened in the police headquarters of Spin Boldak district of Kandahar province, provincial government spokesman Jaweed Faisal told AFP, and provincial police chief General Abdul Razik said it came after an argument.

It takes the toll among international coalition soldiers from insider attacks to 10 in less than two weeks, sharply eroding trust between foreign troops and the Afghans they work with.

Two American soldiers were killed in an incident on Friday, a week after six were killed in a single day on August 10. Another NATO soldier was killed three days earlier.

The total green-on-blue toll makes up 13 percent of all international coalition deaths this year, according to figures from NATO and a tally of total deaths kept by the website.

Dempsey suggested the Afghan government could be doing more to thwart the insider attacks, including more lower-level officials speaking out on the problem.

"I do think the more they can be as concerned about it as we are, the better off we'll all be," he said.

In another a sign of US concern, on Saturday Defense Secretary Leon Panetta urged Afghan President Hamid Karzai to bolster cooperation with ISAF forces to contain the insider threat, according to the Pentagon.

Measures should include improved intelligence and more rigorous vetting of Afghan recruits, a statement said.

NATO has some 130,000 troops in Afghanistan who are due to pull out in 2014 and are spending increasing amounts of time working alongside and training Afghan forces who will take over when they leave.

Dempsey said the insider violence would not alter the timetable for withdrawal.

But the growing number of insider attacks is likely to add to pressure in NATO nations for an exit as soon as possible from the increasingly unpopular war, now nearly 11 years old.

Photo AFP, Johannes Eisele