DERA ISMAIL KHAN, Pakistan -- A suspected American missile strike killed five alleged militants in northwestern Pakistan early Thursday, an intelligence official said, the fourth attack in 24 hours as the U.S. steps up the tactic to keep Al Qaeda and its allies under pressure.

The barrage against houses and a vehicle in the mountains close to the Afghan border was one of the most intense since the attacks were stepped up more than two years ago. Most are believed to be fired from unmanned, remote-controlled planes that can hover for hours above the area.

U.S. officials do not publicly acknowledge the strikes but have said privately that they have killed several senior Taliban and Al Qaeda militants and scores of foot soldiers in a region largely out of the control of the Pakistani state. Critics say innocents are also killed, fueling support for the insurgency.

The latest attack took place before dawn on a house close to a disused match factory a little more than a mile (three kilometers) west of Miran Shah town, a hub for local and international militants in the North Waziristan region, an intelligence official said. He spoke on condition of anonymity in line with the policy of his agency. Five alleged militants were killed, he said.

The three attacks Wednesday also took place in North Waziristan, a lawless region home to insurgents battling foreign troops just across the border in Afghanistan, Al Qaeda leaders plotting attacks in the West and extremists behind bombings in Pakistan. There have been at least four other attacks over the last week.

Pakistani intelligence officials working from army bases in North Waziristan have a network of spies who inform them of the attacks. Sometimes journalists are able to speak by phone to villagers who witness them. Pakistan security agencies are believed to cooperate with at least some of the strikes, but there is very little independent reporting of them because the region is so dangerous for outsiders.

The names of those killed are rarely released, and allegations of civilian casualties are not publicly investigated.

The militants have stepped up their own attacks in Pakistan in recent days, just as the army focuses on helping millions of victims from the worst floods in the country's history. Four big bombs have killed at least 135 people in less than a week.

Pakistan's army has launched several offensives in the northwest over the last two years, but has resisted moving into North Waziristan despite U.S. pressure. A major militant faction there, the Haqqani network, is blamed for attacks against U.S. troops in Afghanistan but has refrained from striking inside Pakistan. Analysts believe the army views the network, with which it has historical links, as an important tool to secure its interests in Afghanistan once foreign troops withdraw.