Rear Admiral Gregory Smith said Pakistani soldiers at a border checkpoint were observed firing on two US OH-58 Kiowa helicopters covering a patrol of Afghan and US troops about a mile and a half inside Afghanistan.

"The ground forces then fired into the hillside nearby that checkpoint, gained their attention, which worked," Smith told AFP by telephone.

"Unfortunately, though, the Pak unit decided to shoot down a hillside at our ground forces. Our ground forces returned fire."

Smith, a spokesman for the US Central Command, said no one on either side was hit in the exchange, which occurred in late afternoon, and the helicopters never fired any rounds.

"The whole thing lasted about five minutes," he said. "It all ended quickly."

The Pakistani military said its troops had fired warning shots at two helicopters which were "well within Pakistani territory."

But Smith and Pentagon officials said the helicopters were in Afghan air space.

The ground unit, which spotted the Pakistanis firing at the helicopters, consisted of a small US training team embedded with an Afghan border police unit, he said.

The two sides did not attempt to communicate during the incident, but the US and Pakistani militaries have since been in communication, Smith said.

"We have to work our way through this so that we don't have any escalation of force issues in the future," he said.

Rising insurgent violence in Afghanistan has tripped alarms in Washington, which has stepped up US missile strikes in the tribal areas and infuriated Islamabad by conducting a cross-border raid earlier this month.

In a speech to the UN General Assembly Thursday, Pakistani President Asif Ali Zardari reiterated that Pakistan would not tolerate violations of its sovereignty.

"Just as we will not let Pakistani's territory to be used by terrorists for attacks against our people and our neighbors, we cannot allow our territory and our sovereignty to be violated by our friends," he told the UN General Assembly.

US Defense Secretary Robert Gates told Congress this week US forces had authority to act in self defense, implicitly acknowledging the cross border operations. But he also stressed that the US military needs Pakistan to be a "willing partner."

The NATO-led International Security Assistance Force in Kabul said Thursday's incident occurred near Tanai district, Khost, which borders North Waziristan, a tribal area where the Taliban and Al-Qaeda are said to have bases.

The Pentagon quickly called Thursday's events "an unfortunate misunderstanding" and confirmed US helicopters were involved.

"They are confident that they were in Afghan air space the whole time," said Bryan Whitman, a Pentagon spokesman.

A State Department spokesman said Washington wanted an explanation from Pakistan, commenting that "the Taliban are not flying helicopters."

"We have been in touch with the Pakistanis about this and we certainly want to have an explanation," the spokesman said.

"These ISAF helicopters were operating inside Afghan territory, and as far as I know, the Taliban are not flying helicopters. So we want to find out what happened and the Pakistanis are trying to give us an explanation."

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