KABUL, Afghanistan — The new U.S. ambassador to Afghanistan called a sudden rash of attacks on international forces by their Afghan partners "troubling" Monday, after an Afghan policeman opened fire on NATO forces in the fifth such assault in a week.

No international service members were killed in the latest attack. A Taliban spokesman claimed responsibility for the shooting in the eastern province of Nangarhar, claiming the attacker was a police officer who had been in contact with insurgents before the assault.

A spike in so-called "green-on-blue" attacks, in which Afghan security forces or attackers wearing their uniforms turn their guns on coalition troops, has raised concerns as NATO aims to turn over control for security to Afghan forces in a little more than two years.

"It's obviously very troubling, not just to us, but it's also very troubling to our Afghan partners," U.S. Ambassador James Cunningham told reporters in the capital in his first public comments after taking over the post earlier in the day.

"There's a lot of work being done to understand why this is happening," Cunningham said. He said it was not clear if all of the attackers were Taliban infiltrators, but noted that the strikes still threaten the "confidence and trust" needed for the two military forces to work together.

"Obviously this undermines or attacks that confidence and trust," Cunningham said.

Taliban insurgents are eager to exploit any such rift.

The trend also raises renewed concern that insurgents may be infiltrating the Afghan army and police, despite intensified screening.

At least seven American service members have been killed in the past week by either their Afghan counterparts or attackers wearing their uniforms.

NATO spokesman Charlie Stadtlander said an initial investigation indicated that Monday's attacker was an Afghan police officer, though the man was wearing civilian clothes.

He said there were no NATO deaths but would not say if any international service members were wounded in the attack, citing coalition policy.

At least one Afghan intelligence agent was wounded in Monday's shooting, according to according to Ahmad Zia Abdulzai, spokesman for the Nangarhar provincial governor.

"The shooter has escaped, and Afghan security forces are looking for him," Abdulzai said.

Taliban spokesman Zabiullah Mujahid issued a statement Monday afternoon praising the shooting. He said the police attacker had been "waiting for such an opportunity to attack" international forces.

Mujahid said three Americans were killed, though the insurgents often exaggerate the results of their attacks and are quick to adopt any incident that allows them to claim support among Afghans.

The coalition on Monday sought to dispel any idea that the rising number of turncoat attacks signals any shift in public sympathy toward insurgents, increased Taliban infiltration or growing resentment toward the mostly American coalition forces.

Insurgents were behind only about 10 percent of this year's reported green-on-blue shootings, a NATO spokesman said, citing investigations into attacks before those of the past week.

"Obviously, the Taliban want to take credit for these things, but the fact of the matter is the majority of these attacks result from individual grievances, cultural misunderstandings or personal or battle stress," said James Graybeal, deputy political affairs officer for the coalition.

He insisted the deadly violence is relatively small scale.

"We're talking about 31 individual bad actors out of a total of almost 340,000 (Afghan security forces)," he said.

Coalition officials say the attacks have not impeded plans to hand over security to Afghan forces – which are supposed to reach a strength of 352,000 in a few months – by the end of 2014.

Green-on-blue attacks are on the rise. So far this year, 34 coalition troops have been killed in 27 attacks, compared to 11 attacks and 20 deaths in 2011, according to an Associated Press count, and five attacks in each of the previous two years.

Six died in two separate attacks on Friday in different areas of the volatile southern province of Helmand.

NATO has said both attackers have been detained, though it has released little information about the shootings, and accounts from other officials differ.

Also Monday, Afghan President Hamid Karzai appointed his former defense minister, Abdul Rahim Wardak, as one of his senior advisers. The move came after parliament voted earlier in the month to dismiss Wardak over allegations of corruption in his ministry, as well as failure to prevent cross-border shelling from Pakistan.

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Associated Press writer Rahim Faiez contributed to this report.