A NATO helicopter incursion into Pakistan's North Waziristan has wounded two security personnel, local intelligence officials say.
Tuesday's incident in the Datta Khel district comes amid tensions between Islamabad and the West following the US raid in Abbottabad in which Osama bin Laden was killed and protests in Pakistan over US drone strikes on its territory.
"It happened early morning," a Pakistani intelligence official in the region, who declined to be identified because he was not authorised to speak to the media, told the Reuters news agency.
"The helicopter hit a Pakistani checkpoint on the border in the Datta Khel area."
Many fighters loyal to al-Qaeda, including foreign ones, are based in Datta Khel, a frequent target of US drone strikes. The place is also a stronghold of fighters loyal to Hafiz Gul Bahadur, a key Pakistani Taliban commander.
The incursion came a day after a drone attack in the same place left seven fighters dead, Pakistani officials said.
One of the dead fighters, an Arab, was the son of an al-Qaeda operative identified as Abu Kashif, an intelligence official said.
Another intelligence official said several Pakistani helicopters took off from Miranshah, the main town in North Waziristan, towards the site of the attack, but the purpose of the Pakistani mobilisation was unclear.
There was no immediate comment from the Pakistani military.
Lieutenant-Commander Colette Murphy, a spokeswoman for the NATO-led International Security Assistance Force (ISAF), said in the Afghan capital, Kabul, that the coalition was looking into the incident. She made no further comment.
North Waziristan is the base of the Haqqani network, a group of fighters allied to the Taliban which NATO says is fuelling the conflict in neighbouring Afghanistan. US-led drone aircraft have repeatedly targeted the area over the past year.
Pakistan's Express television channel said the checkpoint targeted in the latest attack was located right on the border between Afghanistan and Pakistan.
It quoted officials as saying that the helicopters were just about to intrude into Pakistani territory when aerial gunshots were fired. They subsequently moved back but fired a retaliatory shot, which caused rubble to fall from the nearby mountains, wounding the security personnel.