Pakistan's military has filed a strong protest after an exchange of fire between its ground forces and NATO helicopters along the Pakistan-Afghanistan border.
Pakistan says troops at a border post opened fire early Tuesday when the helicopters violated Pakistani airspace. It says two soldiers were wounded in the exchange of fire.
NATO confirms it had helicopters flying near the border, and said it is investigating the incident.
A Western military official in Kabul said the helicopters were on the Afghan side of the border and received fire from inside Pakistan. He said one of the helicopters shot back only after coming under fire twice.
Tuesday's exchange near the Datta Khel area of North Waziristan came amid heightened tensions between the United States and Pakistan, following the U.S. raid that killed al-Qaida leader Osama bin Laden in the Pakistani city of Abbottabad.
U.S. Senator John Kerry was in Pakistan Monday on a visit aimed at repairing relations. On Tuesday, he chaired a Senate Foreign Relations Committee hearing on ties with Pakistan.
A former U.S. national security adviser told the panel in Washington that Pakistan must change its behavior if frayed ties are to be salvaged.
Retired Marine General James Jones said Pakistan has tough choices to make, and needs to show the world that it will not tolerate the existence of terrorist organizations on its soil. He said what happens in coming weeks will have significant "strategic consequences" for the future U.S.-Pakistani relations. He said that when it comes to mending fences, more responsibility is on Pakistan.
Senator Kerry, a Democrat, said the United States has "100,000 reasons" to maintain a productive relationship with Pakistan - a reference to America's troop commitment in neighboring Afghanistan.
The committee's senior Republican, Senator Richard Lugar warned that the U.S. does not give out "blank checks," referring to billions of dollars in aid given to Pakistan. He said Pakistan must do "much more than it has" to root out terrorists in the country.
Separately on Tuesday, Pakistani security forces say they foiled a suicide attack in the capital of southwestern Baluchistan province, Quetta. Officials say five militants, including three women, were planning to attack an army checkpoint when police intercepted them and killed all five of them.
Some information for this report was provided by AFP and AP.