Afghanistan's president has warned President Barack Obama that civilian casualties like the nine Afghan boys killed by coalition helicopters this week are a serious problem that needs to be better addressed by the U.S.-led forces.

President Hamid Karzai and Obama spoke over a secure video conference link on Wednesday, the same day NATO forces acknowledged accidentally killing the boys, ages 12 and under.

Civilian casualties have long been a source of friction between the NATO force and Karzai, who condemned the deaths, saying the victims were "innocent children who were collecting firewood for their families during this cold winter."

"The Afghan president said the increase in civilian casualties is a serious difficulty and he emphasized the need to make progress on this," said a statement released Thursday by Karzai's office. "The American president expressed strong regret for recent civilian casualties, and said he has told his military officials to work to decrease civilian casualties."

Civilian deaths have jumped this year primarily because of increased attacks from insurgents, according to U.N. data. In the latest such incident, an Afghan civilian was killed late Wednesday by a roadside bomb outside southern Kandahar city, the provincial governor's office said.

But allegations of civilian deaths from NATO forces who pledge to protect the population often cause much more anger.

Karzai's relations with the Obama administration have been strained compared with the personal relationship he had with former President George W. Bush, who had frequent videoconference calls with the mercurial Afghan leader.

Last month, Afghanistan's new ambassador to the United States, Eklil Hakimi, proposed more regular communication between Obama and Karzai, saying the more they interact face-to-face by phone or videoconference calls, the better U.S.-Afghan relations will be.

On Wednesday, U.S. Defense Secretary Robert Gates told Congress that American officials have hurt the war effort by often not taking seriously Karzai's complaints on issues ranging from civilian casualties to private security companies.

The killing of the nine boys took place Tuesday in the Pech valley area of Kunar province. Several hundred villagers in the area responded by protesting for four hours Wednesday against coalition strikes. Demonstrators chanted "Death to America" and "Death to the spies," a reference to what they said was bad intelligence given to helicopter weapons teams.

The incident came less than two weeks after tribal elders there claimed NATO forces killed more than 50 civilians in recent air and ground strikes. An Afghan government investigation maintained that 65 civilians died in recent coalition operations in a remote part of the province, a hotbed of the insurgency.

The coalition denied allegations that 65 civilians were killed in Kunar, saying video clearly showed troops targeting and killing dozens of insurgents.

NATO apologized for the boys' deaths on Wednesday. Gen. David Petraeus, top commander of coalition forces in Afghanistan, said disciplinary action, if warranted, would be taken.

On Thursday, Lt. Gen. David Rodriguez, who directs day-to-day operations of coalition forces across Afghanistan, issued a video statement of apology.

In the video, Rodriguez said that that troops at the base were responding to a rocket attack and dispatched attack helicopters to the location they were told the rockets came from.

"The helicopters identified what they thought were insurgents, killing nine," Rodriguez said.

A short time later, the coalition learned the nine were not insurgents, but boys who had been cutting wood, he said. Rodriguez said the coalition has been working to prevent civilian casualties, but "we acknowledge we have to do better."

A NATO service member was killed Thursday by a bomb attack in southern Afghanistan, the international military coalition said. NATO did not provide further details or the victim's nationality, in keeping with a policy of waiting for national authorities to identify casualties.

Thirty-six NATO service members were killed in Afghanistan in February.


Associated Press Writer Mirwais Khan contributed to this report from Kandahar, Afghanistan.