U.S. Defense Secretary Leon Panetta said Monday that while he is very concerned about rogue Afghan troops and police turning their guns on U.S. and allied forces, he sees the insider attacks as the "last gasp" of a Taliban insurgency that has not been able to regain lost ground.

The defense chief's comments followed unusually sharp criticism from Army Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff.

Dempsey on Sunday called the escalating insider attacks a "very serious threat" to the Afghanistan campaign.

Dempsey vowed that something has to change in order to address the escalating problem, and he suggested that the Afghans need to take the matter as seriously as the Americans do.

There have been three so-called insider attacks against U.S. and NATO troops over the past several days, including an attack at a checkpoint by Afghan police Sunday that killed four American soldiers.

More than 50 international service members have died at the hands of their Afghan allies or those who have infiltrated their ranks so far this year. At least 12 such attacks came in August alone, leaving 15 dead.

The spike in insider attacks is souring the relationship between NATO troops and the Afghan forces that they are training and fighting alongside. But military and defense leaders have insisted that these attacks are not hampering the war effort, and that it will not impact the plans to have combat troops out of Afghanistan by the end of 2014.

The U.S. is expected to announce in the next day or two that it has completed the withdrawal of the 33,000 troops ordered into the fight as part of a military surge three years ago.

With those troops gone, the U.S. will have 68,000 forces on the ground in Afghanistan.

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"We're all seized with (the) problem," said Dempsey, after discussing the issue at a meeting in Romania with NATO officials. "You can't whitewash it. We can't convince ourselves that we just have to work harder to get through it. Something has to change."

A weekend of deadly attacks began Friday night, when 15 insurgents disguised in U.S. army uniforms killed two Marines, wounded nine other people and destroyed six Harrier fighter jets at a major U.S. base in the south, military officials said. On Saturday, a gunman in the uniform of a government-backed militia force shot dead two British soldiers in Helmand province in the southwest.