A Nato helicopter based in Afghanistan intruded into Pakistan's North Waziristan region on Tuesday, wounding two troops, local intelligence officials said, adding to tensions between Islamabad and the west.
"It happened early morning," a Pakistani intelligence official in the region, who declined to be identified, told Reuters. "The helicopter hit a Pakistani checkpost on the border in the Datta Khel area."
Another intelligence official said several Pakistani helicopters took off from Miranshah, the main town in North Waziristan, towards the site of the attack. The purpose of the Pakistani mobilisation was unclear.
The Pakistani military was unavailable for immediate comment.
Lieutenant Commander Colette Murphy, a spokeswoman for the Nato-led International Security Assistance Force (Isaf), said in Kabul that the coalition was aware of the reports and was looking into them.
North Waziristan is the base of the Haqqani network blamed for the insurgency in eastern Afghanistan. US-led drone aircraft have repeatedly targeted the area over the past year.
Many militants, including foreign fighters loyal to al-Qaida, are based in Datta Khel. It is a stronghold of fighters loyal to Hafiz Gul Bahadur and has been a frequent target of US drone strikes .
Pakistan's Express 24/7 television channel said the checkpost was 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 released rubble from the nearby mountains and wounded the security personnel.
The reported incursion came a day after unmanned US drone aircraft fired missiles in Datta Khel, killing 12 militants, Pakistani officials said.
An intelligence official said that one of the dead militants, an Arab, was the son of an al-Qaida operative identified as Abu Kashif. There was no way to verify the death toll. Militants often dispute official accounts of drone attacks.
Relations between the US and Pakistan have been pushed almost to breaking point after the secret 2 May raid on Abbottabad that killed Osama bin Laden, embarrassing and humiliating the powerful Pakistani army and intelligence service.
In late January, the undercover CIA contractor Raymond Davis killed two Pakistanis in Lahore, resulting in a six-week standoff over diplomatic immunity. While on 30 September two Pakistani soldiers were killed and four wounded when Nato helicopters crossed the border while pursuing insurgents. Pakistan retaliated by shutting down the supply route for Nato troops in Afghanistan.
Washington sees the rugged border region between Afghanistan and Pakistan as a critical battleground against al-Qaida and the Taliban.