By Syed Shoaib Hasan
BBC News, Islamabad
At least six soldiers have been killed in an ambush by pro-Taleban militants in the Pakistani tribal region of North Waziristan, military officials say.
North Waziristan has been the scene of fierce clashes this year
The attack left 20 soldiers seriously injured, they added.
The military said it was the heaviest fighting in the troubled region since October 2007, when an air offensive left hundreds of militants dead.
North and South Waziristan have been at the centre of fighting between the army and Taleban and al-Qaeda militants.
Rocket and gunfire
Officials say that in the latest violence a military convoy was ambushed on its way from the town of Miranshah to nearby Razmak.
Local officials say the six soldiers were killed in a hail of rocket fire and gunfire. A military spokesman said that helicopter gunships chased and killed 15 of the assailants, but this could not be independently confirmed.
In an interview with the BBC, a militant spokesman denied any casualties.
The violence comes days after the military declared victory in an operation against militants in the Swat valley.
The valley is governed by parliament in Islamabad, unlike the tribal areas which enjoy much more autonomy.