PESHAWAR, Pakistan (Reuters) - The leader of the Pakistani Taliban, the militant movement that poses the gravest security threat to the country, is believed to have been killed by a U.S. drone strike, four Pakistan intelligence officials told Reuters on Sunday. U.S. officials, speaking to Reuters on condition of anonymity, could not confirm his death. But it may not ease violence in the long term in Pakistan, which is seen as critical for U.S. efforts to fight global militancy, most crucially in neighboring Afghanistan. For more stories on Pakistan see link.reuters.com/kac58m The death of Hakimullah's predecessor, Baitullah Mehsud, in a drone strike in 2009 raised false hopes that the Tehrik-e-Taliban Pakistan, or TTP, could be broken. The Pakistani Taliban said Hakimullah was still alive, but the denial was far less assertive than one issued in 2010 after media reports said he had been killed in a drone strike. He is a holy warrior and we will wish him martyrdom," said TTP spokesman Ihsanullah Ihsan. The TTP launched an insurgency in 2007 after the military began a major crackdown on militants. Fighters were particularly incensed when Pakistani security forces stormed the Red Mosque complex run by hard-line clerics in the capital, Islamabad. The government said 102 people were killed in fighting in the incident. The TTP delivered on threats to carry out revenge attacks in Pakistan after U.S. special forces killed al Qaeda leader Osama bin Laden in a secret raid in a Pakistani town in May last year. More recently, some senior Taliban commanders said the umbrella group had started exploratory peace talks with the government. But it is not clear if all factions were on board. Hakimullah was not only in danger of being killed by the drone campaign that President Barack Obama has escalated, or by Pakistani military operations. He and his powerful deputy, Wali-ur-Rehman, were at each other's throats and hostilities were close to open warfare, Taliban sources say. Al Qaeda and the Taliban in Afganistan have been trying to sort out differences between Pakistani Taliban commanders so they can aid their fight against U.S.-led NATO forces across the border in Afghanistan. Any division within the TTP could hinder the Afghan Taliban and al Qaeda's struggle in Afghanistan against the United States and its allies, making it tougher to recruit young fighters and disrupting safe havens in Pakistan that Washington says are used by the Afghan militants. Hakimullah, who has a sharp face framed by shaggy hair and a disarming grin, is considered to be one of the most ruthless Taliban commanders. He is also ambitious. Under his leadership, the Taliban has vowed to expand its violent campaign overseas to hit Western targets. A suicide bombing at a U.S. base in Afghanistan's Khost province in 2009 killed seven CIA employees. In video footage released after the attack, the bomber was shown sitting with Hakimullah Mehsud. Shortly afterwards, the United States added the TTP to its list of foreign terrorist organizations and set rewards of up to $5 million for information leading to Hakimullah Mehsud or Wali-ur-Rehman. A Pakistani-born American who tried to set off a car bomb in New York's Times Square in 2010 told a U.S. court he received bomb-making training and funding from the Pakistani Taliban. Additional reporting by Mark Hosenball and Phil Stewart in Washington; Writing by Michael Georgy; Editing by Peter Graff and Peter Cooney