By Syed Shoaib Hasan
BBC News, Islamabad
Weapons are easily available in Pakistan's tribal territories
Some 40 people are reported to have been killed in sectarian violence in Pakistan's North West Frontier Province (NWFP) that began on Wednesday.
The violence follows rising tensions between the Sunni Muslim Orakzai tribe and the minority Shia Katchai tribe.
Heavy weapons were used in the clashes in the Lautang area of Kohat district, a local official told the BBC.
Separately five people were killed in an attack on an ambulance in the Kurram tribal agency.
Sectarian violence has killed thousands in Pakistan over the years.
In the north-west, traditional rivalries have been exacerbated by an army operation against pro-Taleban militants.
Mehtab Khan, a senior administration official in the Kohat district, told the BBC "heavy weapons" had caused "massive damage" to houses in the Lautang area.
Mr Khan said many of the dead were women and children and their bodies had not been recovered because the violence had yet to abate.
Seventeen more people were killed in Lautang on Thursday, after more than 20 people were killed on Wednesday, officials say.
The violence began at the weekend when members of an Orakzai tribal council were attacked in an area controlled by the Katchai tribe. That clash left nearly 20 people dead.
Locals officials say at least 10 branches of the Orakzai tribe are surrounding the Katchai region in response.
They say they fear a massacre if the government does not intervene.
The authorities initially tried to negotiate an end to the issue after the attack on the tribal council.
Reports now say the army has sent helicopter gunships in the area.
In a separate incident, five people have been killed in an attack on an ambulance in the Kurram tribal agency close to the Afghan border.
The authorities say the attack near the town of Tall is also sectarian in nature.
"The ambulance was coming from Parachinar, when it was attacked," local police chief Mirzaly Khan told the BBC.
"Several men armed with grenades and heavy weapons attacked the vehicle," Mr Khan said.
Five people, including a security guard and a woman, were killed, he said.
Parachinar and Tall are located in the Kurrum tribal agency, which has a long history of sectarian violence.