WASHINGTON: The Pentagon said on Tuesday it was expanding counterintelligence staff in Afghanistan after a rise in insider attacks by Afghans thought to be friendly to US forces but who have killed 37 coalition troops so far this year.

Last Friday, six US troops were killed in two separate incidents, one which saw an Afghan police commander and several of his men kill three US Marines after inviting them to a Ramadan breakfast to discuss security.

Defense Secretary Leon Panetta said he was deeply concerned by the killings “because of the lives lost and because of the potential damage to our partnership efforts.”

Panetta said more intelligence officers will be added to the larger military units to help root out possible attackers, and officials will do more in-depth forensic reviews of the incidents, in which Afghan Army or police members _ or militants in disguise, attack and kill American troops and other allies.

The increasing number of incidents have eroded trust between the allies just as Nato combat troops prepare to hand over security control to Afghan forces by 2014.

General Martin Dempsey, chairman of the US military's Joint Chiefs of Staff, said at the same news conference that the US military was bolstering counterintelligence expertise at the battalion level and above in Afghanistan.

He said the same was occurring within Afghan forces, which he said had so far discharged hundreds of soldiers who showed a risk of radicalization, including travel back and forth to Pakistan, where many militants enjoy safe haven.

Dempsey said Gen. John Allen, the top US commander in Afghanistan, has created a new team that will review the attacks to determine whether the recruiting process was followed properly, including the required vetting of the Afghan security force members, and whether there were indicators of possible problems that were missed.

So far in 2012, there have been 29 attacks, resulting in 37 coalition deaths, 21 of which were among US forces. For the same period last year, there were 16 attacks and 28 deaths.

In all of 2011, there were 35 coalition troops killed, 24 of whom were US troops.