WASHINGTON -

U.S. and Pakistani forces exchanged gunfire yesterday along the Afghanistan border, as simmering tensions over U.S. incursions into Pakistan reached a full boil.

The incident began, U.S. officials said, when forces from a Pakistani outpost fired on two Kiowa OH-58 reconnaissance helicopters. That touched off a five-minute small-arms fight when a ground unit made up of Afghan and U.S. forces returned fire on the Pakistani outpost, U.S. military officials said.

There were no casualties, U.S. officials said. The shots fired by the U.S.-Afghan unit were "suppressive" fire intended to force the Pakistanis to take cover and stop shooting. The helicopters did not fire any shots, U.S. officials said.

U.S. military officers said the helicopters, under the command of NATO forces, were always within Afghanistan. But Pakistani officials said the helicopter entered Pakistan's tribal regions.

Although Pakistani officials have reported firing on U.S. helicopters and unmanned aircraft in recent weeks, yesterday's incident is the first time U.S. officials have confirmed an attack by Pakistani forces.

Anger in Pakistan has been steadily rising since the U.S. began conducting additional cross-border commando raids as well as stepping up the number of attacks from unmanned Predator drones inside the tribal areas.

American officials do not believe senior Pakistan army officers have issued formal orders to fire on U.S. military units that cross the border. Defense officials said they might never know why the Pakistani forces fired.

The two sides' accounts differed sharply. U.S. military officials said the helicopters were flying in support of the small ground force that was conducting "routine operations" on the Afghanistan border.

Pakistani officials said that the choppers entered deep into Pakistani airspace and flew over North Waziristan for 20 minutes. Residents said that unmanned spy planes were still hovering over the area.

The Pakistani army's Inter Services Public Relations agency said that two helicopters from Afghanistan crossed into Pakistani territory near the Ghulam Khan sector, prompting warning shots from the outpost.

U.S. officials insisted that their forces never left Afghanistan's Khost province and denied the helicopters fired on the Pakistani outpost, saying the only gunfire came from American-led ground forces.

At the United Nations, Pakistani President Asif Ali Zardari said his military fired only flares at the aircraft. In an address to the General Assembly, Zardari said, "We cannot allow our territory and our sovereignty to be violated by our friends."

Tensions spiked after a Sept. 3 raid into South Waziristan by U.S. special operations forces, which prompted protests and a formal condemnation by the Pakistani parliament. A week later, Ashfaq Kayani, the Pakistani army chief of staff who has been courted by Pentagon officials, criticized the U.S. operations. A Pakistani army spokesman, Maj. Gen. Athar Abbas, told the Associated Press that troops should "open fire" to prevent military forces from crossing the border. U.S. officials do not believe Abbas' remarks represented official policy.

In yesterday's incident, western military officers in the region reported that the helicopters stayed about a mile inside the Afghanistan border. But there are frequent disagreements about the precise location of the border.

Bryan Whitman, a Pentagon spokesman, said the incident was a misunderstanding. But he suggested that the Pakistanis should have had little doubt at whom they were firing.

"We know the challenges on the border, but it is not too difficult to imagine who is flying helicopters there," he said. "Pakistan is an ally, not an enemy."

After the incident, U.S. military officials in the region scrambled to meet with Pakistani military liaisons.