Militants forced to flee their havens in Pakistan's mountainous tribal areas are establishing new, smaller cells in the heart of the country and have begun carrying out attacks nationwide, U.S. and Pakistani officials say. The spread of fighters is an unintended consequence of a relatively successful effort by the United States and Pakistan to disrupt the insurgents' operations, through missiles strikes launched by unmanned CIA aircraft and a ground offensive carried out this fall in South Waziristan by the Pakistani army. American and Pakistani officials say the militants' widening reach has added to the challenge for both nations' intelligence, which must now track an insurgent diaspora that can infiltrate Pakistan's teeming cities and blend seamlessly with the local population. A Pakistani intelligence official said the offensive had put militants 'on the run' but added: 'Now they're all over -- Afghanistan, North Waziristan and inside Pakistan.' 'They have scattered their network and structure,' said Muhammad Amir Rana, director of the Pak Institute for Peace Studies, a security-oriented think tank. 'It's easy for many of them to hide in Punjab or Karachi.' Pakistani officials insist that they are doing as much as they can to counter the extremist threat and that they are paying the price. In recent months, militants have unleashed a wave of attacks in Punjab province, the military's home base, with many of the strikes carried out by fighters who have left the Federally Administered Tribal Areas as the pressure there has mounted. But the flow of militants out of the tribal areas has frustrated U.S. intelligence, which escalated the missile strikes using drone aircraft this year but has found targets increasingly scarce in recent months. Because of the Pakistani government's opposition, the CIA has not expanded its campaign of drone warfare beyond the lawless tribal belt in the northwest that hugs the Afghan border. The United States has threatened to enlarge the scope of its drone campaign unless Pakistan steps up its efforts against insurgent groups that have found sanctuary in the country and that focus on attacking U.S. troops in Afghanistan. But with anti-Americanism on the rise in Pakistan, drone attacks outside the tribal belt could elicit a powerful public backlash and could jeopardize Pakistani military cooperation, officials here say. Until this week, the pace of reported drone strikes in the tribal areas had been off sharply from summer highs. Although 2009 has set a record -- 50 drone strikes, compared with 31 last year -- the tempo declined this fall from six or seven per month to about two, according to a tally by the nonprofit group Long War Journal. In addition, until last week, there had been a three-month lull in reported deaths of senior al-Qaida or Taliban operatives. This week, however, has brought a surge in strikes: Suspected Predator drones killed six people Friday, and a barrage of as many as 11 missiles on Thursday killed 16. The strikes, all in North Waziristan, came just days after top U.S. military officials visited Pakistan and urged the government to broaden its offensive into that area. Pakistan declined, saying such a move would stretch its military too thin. Pakistani officials complain that although the drone strikes help incite insurgent attacks against domestic targets, the United States generally does not go after militants who focus their firepower inside Pakistan. Instead, the officials say, the drones are trained on those Taliban and al-Qaida commanders who are most troublesome for U.S. commanders in Afghanistan. U.S. intelligence officials refuse to comment on drone strikes in Pakistan, or even to publicly acknowledge CIA involvement in such flights. Still, American intelligence officials said the drop in reported incidents this fall does not indicate any slackening in the intensity of U.S. efforts to strike al-Qaeda and its allies in the tribal region. 'There's been no decision by anyone to reduce any aspect of counterterrorism operations. That certainly includes the most effective activities,' said a U.S. counterterrorism official familiar with the effort. Yet several counterterrorism experts and former intelligence officials acknowledged that finding targets has become more difficult in recent months. They said militants have adapted their tactics, improved security and executed anyone suspected of being an informant. They also acknowledged that some jihadists from the tribal belt have moved to urban areas, apparently to escape the threat of drones. The Pakistani intelligence official, who spoke on the condition of anonymity, said the Taliban and al-Qaida have ruthlessly purged anyone in their organizations accused of being a spy. The markets in Peshawar, capital of the North-West Frontier Province, are full of DVD recordings of beheadings of suspected informants. Just in North Waziristan, the official said, Pakistani intelligence agencies have lost 30 undercover operatives this year. As a result of the killings, he said, 'quite a few areas have literally become black holes for us.' Although the Pakistani government officially opposes the drone campaign, it cooperates behind the scenes, sharing intelligence with the CIA. In addition to killing suspected spies, Taliban and al-Qaida commanders have destroyed communication towers, stopped talking on the phone and begun to move only at night -- all in an effort to avoid detection. 'They have truly gone underground,' said Ashraf Ali, director of the FATA Research Center. 'Before they were openly roaming the streets. They would hold meetings. But in the daytime now, they can't be seen.' Such tactics do not neutralize the value of the drones, but they are 'understandably effective in depriving the United States of the target-rich environment that existed when we first ramped up the attacks,' said Bruce Hoffman, a terrorism expert and professor at Georgetown University's Edmund A. Walsh School of Foreign Service. A former senior U.S. intelligence official said militant groups have concluded over the past year that 'cities are safer, particularly the big cities, where there's anonymity but also support networks, communication, everything they need.' But the former official also acknowledged a debate within the U.S. counterterrorism community over the balance between remote-control drone strikes and other kinds of operations that are more effective at gathering intelligence. 'If you're killing people,' he said, 'you're not developing sources or informants.' -- Warrick reported from Washington. 