NATO worker killed by woman in Afghan police uniform

NEW: Monday's incident is the first to involve a female shooter NEW: On Sunday, a police commander killed five officers Officials do not know if the woman is indeed part of the Afghan police A Pentagon report says there has been an overall increase in "insider attacks"

Kabul, Afghanistan (CNN) -- A woman in an Afghan police uniform shot and killed an American contractor in the Kabul police headquarters early Monday morning, authorities said.

The shooting death comes a day after five policemen were killed by their commander in northern Jawzjan province.

The incidents add to the rising number of insider attacks by Afghan soldiers and police officers -- or attackers dressed like them.

More than 50 people have been killed in Afghanistan in similar attacks this year, which the Afghan government calls acts of terrorism.

Monday's incident was unusual because it was the first time that such an attack involved a female shooter, said Hagen Messer, a spokesman for NATO's International Security Assistance Force.

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Officials have not yet confirmed whether the woman was indeed a member of the Afghan police force, said Sediq Sediqqi, a spokesman for the Afghan interior ministry. She was arrested.

The victim was a civilian contractor for ISAF who was working as an adviser to Afghan police, said Maj. Martyn Crighton, another ISAF spokesman. A coalition official, who was not authorized to speak on the record, said the contractor was an American.

A bi-annual Pentagon report to Congress this month said there's been an overall increase in "insider attacks" on U.S. or coalition training forces.

"The rise in insider attacks has the potential to adversely affect the coalition's political landscape," according to the report. "It remains clear that the insider threat is both an enemy tactic and has a cultural component," according to the report.

No group immediately claimed responsibility for Monday's attack.

But a Taliban spokesman said it carried out Sunday's attack on the five policemen.

In that incident, the police commander who killed the men was a Taliban insurgent who had infiltrated the Afghan police, said Abdul Aziz Ghairat, police chief of Jawzjan province.

The Taliban spokesman said the commander was in touch with the militant group before the attack, and is now in a safe place in their midst.

The Pentagon report said Taliban insurgents have lost some of their punch since their 2010 peak but they remain "resilient and determined" and "will likely attempt to regain lost ground and influence" through assassinations, high-profile attacks, the use of roadside bombs and other violence.

Also Monday, an ISAF service member was killed in an insurgent attack in eastern Afghanistan.

Per policy, ISAF did not release the service member's name or nationality.