KABUL, Afghanistan – Pakistani troops fired at American reconnaissance helicopters patrolling the Afghan-Pakistan border Thursday, heightening tensions as U.S. steps up cross-border operations in a region known as a haven for Taliban and al-Qaeda 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.

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

U.S. forces and Pakistan's military “are working together to resolve the matter,” a NATO statement said.

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.

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.

Pakistan's military spokesman Maj. Gen. Athar 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, a U.S. official said the U.S. coalition in Afghanistan immediately demanded an explanation from Pakistan, the official said on condition of anonymity because he was not authorized to speak on the record.

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.

Associated Press reporter Pauline Jelinek contributed to this report from Washington.