DERA ISMAIL KHAN, Pakistan — Three suspected U.S. missile strikes in less than 12 hours hit militant targets in northwestern Pakistan on Wednesday, officials said, an unusually intense barrage that follows four other such attacks in the last week. At least 14 suspected militants were killed.

The strikes were 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.

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.

The United States has fired hundreds of missiles into northwest Pakistan over the last 2 1/2 years. American officials do not publicly acknowledge such strikes, but have said privately that they have killed several senior Taliban and al Qaeda militants and scores of foot soldiers.

Critics say innocents are also killed, fueling support for the insurgency.

The first attack was on a house in the village of Dande Darpa Khel near Miran Shah, the main town in North Waziristan, two Pakistani intelligence officials said on condition of anonymity because they were not authorized to talk to the media.

The house was owned by Maulvi Azizullah, a member of the Haqqani network, a militant group based in North Waziristan that U.S. military officials have called the most dangerous threat to NATO troops in Afghanistan. Six militants were killed, they said.

The second missile hit a car traveling a few kilometres from the border, killing four people associated with the Haqqani network, officials said. Zameedullah Wazir, a resident of Ambar Shaga, said he and others tried to get close to the vehicle, but were told to leave by Taliban fighters who arrived soon after in three vehicles.

After nightfall, four other militants were killed, again in a Haqqani-dominated area close to Miran Shah, in an attack on a house, officials said.

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. The Haqqani network has refrained from launching attacks inside Pakistan, and analysts believe the army views them as an important tool to secure its interests in Afghanistan once foreign troops withdraw.

Without a Pakistani military offensive, the U.S. has had to rely on drone strikes to battle the group.

The Pakistani government has publicly criticized the missiles strikes as violations of its sovereignty, but is believed to help the CIA carry out the attacks, especially when they target militants at war with Pakistan.