KABUL, Afghanistan—Pakistani troops fired at American reconnaissance helicopters near the Afghan-Pakistan border Thursday, and ground troops then exchanged fire, the U.S. military said.

No injuries were reported, but the incident heightened tensions as the U.S. steps up cross-border operations in a volatile region known as a haven for Taliban and al-Qaida militants.

Two American OH-58 reconnaissance helicopters, known as Kiowas, were on a routine afternoon patrol in the eastern province of Khost when they received small-arms fire from a Pakistani border post, said Tech Sgt. Kevin Wallace, a U.S. military spokesman. There was no damage to aircraft or crew, officials said.

U.S. Central Command spokesman Rear Adm. Greg Smith said Pakistan and American ground troops exchanged fire after Pakistani forces shot at the helicopters.

He said a joint patrol of Americans and Afghan border police was moving about a mile and a half inside Afghanistan with the helicopters above them. The ground troops reported that Pakistani forces fired toward the helicopters and when they saw that happen, they fired off suppression rounds toward the hilltop. They did so, Smith said from Centcom headquarters in Tampa, Fla., “to make certain that they (the Pakistanis) realized they should stop shooting.”

The Pakistani border patrol forces then shot back down on the joint location of the U.S.-Afghan patrol. “The whole thing lasted five minutes,” Smith said. The Pakistani military, however, said its troops fired warning shots after the helicopters crossed “well within” Pakistani territory.

“On this, the helicopters returned fire and flew back,” the Pakistani military said in an English-language statement.

And in New York, Pakistan’s new president, Asif Ali Zardari, said his military fired only “flares” at foreign helicopters that he claimed strayed across the border from Afghanistan.

Zardari said his forces fired only as a way “to make sure that they know that they crossed the border line.” “Sometimes the border is so mixed that they don’t realize they have crossed the border,” he told reporters before he began a meeting with Secretary of State Condoleezza Rice.

The Pakistani military said the matter was “being resolved” in consultations between the army and the NATO force in Afghanistan. A NATO statement said the militaries were “working together to resolve the matter.”

The U.S. has stepped up attacks on suspected militants in the frontier area, mostly by missiles fired from unmanned drones operating from Afghanistan. The incursions—especially a ground raid into South Waziristan by American commandos Sept. 3—have angered many Pakistanis. Pakistani army spokesman Maj. Gen. Athar Abbas said last week that Pakistani field commanders have previously tolerated international forces crossing a short way into Pakistan because of the ill-defined and contested nature of the mountainous frontier.

“But after the (Sept. 3) incident, the orders are clear,” Abbas said. “In case it happens again in this form, that there is a very significant detection, which is very definite, no ambiguity, across the border, on ground or in the air: open fire.”

On Wednesday, Pakistan’s army said it had found the wreckage of a suspected surveillance drone in South Waziristan, but denied claims by Pakistani intelligence officials that troops and local people shot down the aircraft.

In Washington, Defense Department spokesman Bryan Whitman said the coalition immediately requested an explanation from Pakistan for what he described as a “troubling” incident.

“It would be fairly hard to mistake a helicopter flying in that region as anything but ISAF or U.S.,” Whitman said.

He said militants have always tried to exploit the border region.

“It’s a challenge along the border and that’s why we continue to look for ways to improve our coordination,” Whitman said.

Asked how Pakistani forces could mistake U.S. helicopters for enemy forces—especially since Taliban and al-Qaida forces don’t have aircraft— Whitman said: “Only Pakistan can articulate their intent.”