Another NATO troop killed by Afghan police officer

Kabul, Afghanistan (CNN) -- Another attacker in an Afghan police uniform killed a member of the NATO forces Sunday in Afghanistan, the latest in a slew of so-called "green-on-blue" attacks.

The incident took place in southern Afghanistan, NATO's International Security Assistance Force said.

The attack brings to 40 the number of NATO troops killed in 32 such attacks this year, according to figures provided by ISAF.

U.S. Secretary of Defense Leon Panetta urged Afghanistan on Saturday to take further steps to vet security force recruits.

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In a telephone call Saturday, Panetta encouraged President Hamid Karzai to work closely with the commander of NATO forces in Afghanistan to counter such attacks, Pentagon spokesman George Little said in a written statement.

There were two green-on-blue attacks Friday. An Afghan police officer opened fire on U.S. troops in Farah province, killing two, the same day that an Afghan soldier wounded two NATO service members and an Afghan security force member in Kandahar province.

Even as NATO blamed the majority of attacks on personal grievances rather than an infiltration by insurgents, ISAF commanding Gen. John Allen ordered all troops in the country to carry loaded weapons around the clock.

Panetta urged Karzai to work closely with Allen to strengthen ISAF-Afghan cooperation to counter the threat, "including augmented counterintelligence measures, even more rigorous vetting of Afghan recruits, and stepped up engagement with village elders, who often play a key role by vouching for Afghan security personnel," Little said.

The attacks come at a critical time for the United States, which has picked up the pace of training Afghan forces as it prepares to withdraw troops by the end of 2014.

The telephone call between the two leaders came the same day a NATO airstrike killed a senior Taliban leader in the Kunar province, along Afghanistan's northern border with Pakistan, ISAF said in a statement released Sunday.

ISAF identified the man as Maulawi Nur Mohammad, whom it accused of planning and directing all Taliban activity in the Chapah Darah district.

"During the operation, the Afghan and coalition security force observed a large group of heavily armed insurgents engaging in insurgent activity," according to the release. "After ensuring there were no civilians in the area, the security force engaged the insurgents with a precision airstrike."

On Sunday, three ISAF service members were also killed in an improvised explosive device attack in eastern Afghanistan, ISAF said.

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CNN's Barbara Starr contributed to this report.