HONOLULU - Afghan President Hamid Karzai recognizes that the United States is a friend and ally even as he warns that civilian casualties in Afghanistan risk making US-led troops appear to be an occupying force, US Defense Secretary Robert Gates said on Tuesday.

Gates, who spoke to reporters in Hawaii while en route to an Asian security conference, was reacting to Karzai's statement on Tuesday that raids on Afghan homes in pursuit of insurgents are "not allowed" and his threat to take action if a spate of civilian casualties does not stop.

Karzai expressed anger over NATO airstrikes on Sunday in southern Helmand Province that killed at least nine people, most of them small children.

"This is a continuing challenge that we face in the kind of war that we are fighting in Afghanistan," Gates told reporters, when asked about Karzai's comments.

"I think joint investigations of these incidents, when they happen, are important so that we can mutually figure out what exactly happened and what, if anything, went wrong," he said.

Gates noted that General David Petraeus, the head of NATO-led forces in Afghanistan, had a good dialogue with Karzai, and that 80 percent of civilian casualties in Afghanistan are caused by the Taliban.

"The Afghan people have put up with 30 years of war, and I think President Karzai is reflecting the pain and suffering the Afghan people have had to endure," Gates said.

"But at the same time I think he also recognizes, and the Afghan people do, that we are their ally, that we are their friend and we are trying to help them develop the capability to protect themselves so that the Afghan people can see an end to this kind of conflict."

US President Barack Obama plans in July to start a gradual drawdown of US troops in Afghanistan.

White House spokesman Jay Carney earlier on Tuesday reiterated US regret for civilian casualties and said the United States would continue its efforts to reduce them in coordination with NATO and Afghan officials.

"We do not operate in a vacuum in that country and will continue to work with President Karzai on this important issue because we take very seriously his concerns and we share them," Carney told reporters in Washington.

Reuters