Pakistani troops fired on U.S. helicopters patrolling eastern Afganistan Thursday, the Pentagon said, adding that it expects an explanation from Pakistani officials.

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.

"They did not cross the border and they did not fire back," Wallace said.

The Pakistani military disputed that assertion, saying its troops fired warning shots when the two helicopters crossed over the border — and that the U.S. helicopters fired back.

"When the helicopters passed over our border post and were well within Paskitani territory, own security forces fires anticipatory warning shots. On this, the helicopters returned fire and flew back," a Pakistani military statement said.

Pakistan's new president said that his military fired only "flares" at foreign helicopters that he claimed had strayed across the border from Afghanistan into his country.

Pakistani President Asif Ali 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.

Abbas said Pakistan's military was awaiting a full report from Afghanistan on Thursday's shooting, but that Pakistani units had "very clear" orders not to fire across the border. "We are getting it investigated," he said.

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

"The Pakistanis have to provide us with a better understanding as to why this took place," Whitman told Pentagon reporters.

He said the 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 Qaeda forces don't have aircraft — Whitman said: "Only Pakistan can articulate their intent."

Pakistani civilian leaders have condemned the cross-border operations by U.S. forces, which have been authorized by President Bush, while the army has vowed to defend Pakistan's territory "at all cost."

"We will not tolerate any act against our sovereignty and integrity in the name of the war against terrorism," Prime Minister Yousuf Raza Gilani told journalists Wednesday. "We are fighting extremism and terror not for any another country, but our own country. This is our own war."

Pakistan's tribal areas have become a breeding ground for Taliban and Al Qaeda militants, who are launching attacks inside Pakistan but also across the border into Afghanistan, where the levels of violence have reached record heights since the ouster of the Taliban from power in the U.S.-led invasion in 2001. More than 4,600 people — mostly militants — have died this year in insurgency-related violence in Afghanistan, and the levels of violence in the eastern Afghanistan are 30 percent higher compared to the same period last year, officials say.

In other violence, a remote-controlled bomb struck a police vehicle Wednesday in Spin Boldak district of Kandahar province, killing two officers, said provincial police chief Matiullah Khan.

The Associated Press and Reuters contributed to this report.