2 U.S. troops killed by Afghan cop

• Taliban leader Mullah Omar has purportedly released a statement
• In the statement, Omar says fighters are having success infiltrating Afghan forces
• It follows news that seven NATO troops were killed in "green-on-blue" attacks
• So-called "green-on-blue" attacks are carried out by Afghans in security uniforms

(CNN) -- A uniformed Afghan police officer turned his weapon on U.S. forces in Farah province Friday, killing two of them before being shot and killed himself, a U.S. military statement said.

The killings are the latest in a series of assaults this year carried out by Afghans clad in security force uniforms.

This attack also follows a Thursday statement by the Taliban's elusive leader boasting that fighters are infiltrating Afghanistan's security forces. The statement said fighters are attacking NATO-led forces on their bases, according to a statement purported to be from Mullah Mohammed Omar.

The statement posted Thursday to militant websites also follows news that six Americans troops and one British soldier were killed in "green-on-blue" separate attacks.

"Many Afghans in the rank and files of the enemy have shown a willingness to help the (Taliban) in a shrewd manner," said the statement obtained by the SITE Intelligence Group, which monitors the activities of militant groups on the web.

"As a result, the foreign invaders and their allies at their military centers and bases are suffering crushing blows by these heroic soldiers."

The hunt for Mullah Omar

CNN can't independently verify the authenticity of the statement, which was released in advance of the Eid al Fitr celebration that marks the end of the Muslim holy month of Ramadan.

The Taliban is known to routinely claim responsibility for attacks and inflate casualty numbers.

Omar also urged employees of the Afghan government to seek out the group's newly established "Call and Guidance, Luring and Integration" departments that have been established throughout the country "so that they may leave the enemy ranks and join the fighters," according to SITE.

Afghan security personnel have killed an estimated 93 NATO soldiers since May 2007, and they have killed at least 31 this year alone across the country, said Bill Roggio, managing editor of the Long War Journal blog, which reports and analyzes terror issues. That figure does not include the two deaths in Farah province Friday.

The frequency of so-called "green-on-blue" attacks prompted Afghan intelligence agents to begin going under cover during recruit training to spot possible extremists, military officials say.

U.S. Army Gen. John Allen, commander of NATO troops in Afghanistan, said coalition officials were working on a new procedure to check the backgrounds of Afghans who sign up for the army or police force.

The U.S. government has offered a reward of up to $10 million for information leading to the arrest of Omar, who took over Afghanistan in the early 1990s and established a hard-line Islamic fundamentalist regime that gave shelter to Osama bin Laden and his al Qaeda terrorist network.

Omar vanished from public view after a U.S.-led coalition ousted the Taliban and their leaders from power in Afghanistan in December 2001 for refusing to hand over bin Laden after the 9/11 terror attacks.

Over the years, he refused to be photographed or filmed, and rarely traveled. As a result, Omar's appearance has remained a mystery to many. Those who had met Omar said he has one eye stitched shut; the result of a wound suffered during a battle with Soviet troops during their occupation of Afghanistan.

There have been a number of conflicting reports over the fate of Omar. As late as last year, the Taliban was denying reports its leader was dead, though he has not been seen in public since he was ousted from power.

CNN's Joe Sterling contributed to this report.