A wing of the Taliban based in a Pakistani tribal area bordering Afghanistan say they have scrapped a peace deal with the government.
The group led by Hafiz Gul Bahadur in North Waziristan withdrew from the deal as the army stepped up its offensive against the Taliban in the north-west.
The announcement comes a day after his men ambushed a Pakistani military convoy, killing 16 soldiers.
The militants signed the peace deal with the army in 2007.
Hafiz Gul Bahadur's group had initially pledged to stay on the sidelines during the continuing operation against the country's top Taliban commander, Baitullah Mehsud.
Neighbouring South Waziristan is where the Taliban commander is said to be based. The army wants to eliminate his network of militants based in the mountainous territory there.
The group said they were abandoning the peace deal because of continued US missile strikes and Pakistan's widening anti-Taliban offensive in the north-west.
Announcing their decision, spokesman Ahmedullah Ahmedi, also said they would now carry out attacks on military targets in the region until the army left and US drones strikes were halted.
PEACE DEALS SINCE 2006
Feb 2006 - May 2007: Deal with Taliban chief Baitullah Mehsud for Sararogha, South Waziristan
Feb 2008 - March 2008: With Baitullah Mehsud over parts of South Waziristan
Sept 2006 - June 2009: With Hafiz Gul Bahadur and Maulvi Sadiq Noor for Miranshah, North Waziristan
Mar 2007 - June 2009: With militant Maulvi Nazir Ahmed for Wana, South Waziristan
May 2008 - June 2008: With radical cleric Maulana Fazlullah for Swat valley
Feb 2009 - April 2009: With Maulana Fazlullah for Swat
Most of the drone strikes have been targeted at Hafiz Gul Bahadur and another tribal leader, Maulvi Nazir.
Both leaders signed the peace deals with the army in 2007.
But Maulvi Nazir also abandoned his deal when he declared war on the Pakistan army two days ago.
The BBC's Syed Shoaib Hasan in Islamabad says the scrapping of the deal leaves the army facing a near impossible task - no one has ever defeated a combined insurgency in the Waziristan area.
Pakistan's army began its military offensive in the Swat valley two months ago after the earlier peace deal the Taliban there broke down.
Separately, Pakistan's prime minister Yusuf Raza Gilani has said that the second and third tier leadership of the Pakistani Taliban have been "eliminated" in the government's offensive against the militant network.
He added that the top leaders would soon meet the same fate.
The full-scale operation against the Pakistan Taliban leadership in their main stronghold in the Afghan border region of South Waziristan has yet to begin, says our world affairs correspondent, Mike Wooldridge.
One issue cited by the army as they prepare the ground is that they want to avoid provoking a wider tribal uprising.