U.S. drones fired missiles at three hideouts in a key militant sanctuary close to the Afghan border Friday, killing 18 suspected insurgents in the latest of a series of strikes conducted this week despite protests from Islamabad, Pakistani intelligence officials said.

The strikes all took place in the North Waziristan tribal area, the target of a planned Pakistani military operation that the U.S. expects in the near future. Hundreds of militants and their family members have streamed out of North Waziristan in the past few days in anticipation of the operation, local residents said.

Washington has long demanded Pakistan target militants holed up in North Waziristan and has welcomed the planned operation in the area. But Islamabad is likely to focus on Taliban militants who have been at war with Pakistan, not those who have been fighting the U.S.-led coalition in neighboring Afghanistan.

In a string of strikes Friday just minutes apart, U.S. missiles slammed into mud brick compounds located several kilometers (miles) from each other in the Shawal Valley, a heavily forested, mountainous area in North Waziristan that serves as one of the key crossing points for militants heading into Afghanistan, Pakistani intelligence officials said.

Eighteen suspected militants were killed and another 14 were wounded, the officials said on condition of anonymity because they were not authorized to talk to the media. It was not immediately clear which militant group was hit.

The U.S. has carried out seven drone strikes in the past week in North Waziristan, ignoring repeated Pakistani protests that they violate the country's sovereignty and international law. The covert CIA attacks have increasingly become a point of public conflict between the two countries, complicating an already troubled relationship that is vital to the outcome of war in neighboring Afghanistan.

Pakistan's Foreign Ministry summoned a senior U.S. diplomat Thursday to protest the strikes, and the ministry's spokesman, Moazzam Ahmad Khan, called the attacks "illegal, unproductive" during his weekly press briefing Friday.

But the reality behind the scenes is more nuanced — Pakistan secretly supported the strikes in the past, and U.S. officials say privately that key members of the government and military still do.

They Americans view the public denunciations of the strikes as a political tool to appease the large number of Pakistanis who disapprove of the missile attacks, and insist they have no intention of holstering a key weapon in the fight against Taliban and al-Qaida militants who threaten the West.

Pakistani officials have asked the U.S. to feed intelligence gathered by drones to Pakistani jets and ground forces so they can target the militants. But American officials say Pakistan has proved incapable or unwilling to target militants the U.S considers dangerous.

Last Saturday, a U.S. drone struck a militant hideout in North Waziristan, killing five allies of a powerful warlord, Hafiz Gul Bahadur, whose forces often attack U.S. troops in Afghanistan. American drones fired a flurry of missiles into the area Sunday, killing 10 suspected militants in two separate strikes. On Tuesday, missiles targeting a vehicle in North Waziristan killed five more suspected militants.

One of the reasons President Barack Obama increased the number of drone attacks in Pakistan when he took office in 2009 was the government's refusal to launch an offensive in North Waziristan against militants who carry out cross-border attacks against American forces in Afghanistan.

U.S. Secretary of Defense Leon Panetta recently said Islamabad plans to launch an operation in North Waziristan in the near future targeting the Pakistani Taliban, who have waged a bloody insurgency against the government in Islamabad for years.

Pakistani military officials have said they will slowly increase pressure on the militants in North Waziristan, rather than conduct a sweeping offensive in the area. North Waziristan is the only area in Pakistan's tribal region where the military has not conducted an offensive.

Analysts have said they doubt Pakistan will target militants in North Waziristan responsible for attacks in Afghanistan because they are not seen as much of a threat to the state. Also, Pakistan has historical links with some of the Afghan militants operating in the area, especially the so-called Haqqani network, and many analysts believe Islamabad sees them as key potential allies in Afghanistan after foreign forces withdraw.

Many of the militants who started fleeing North Waziristan in vehicles on Thursday were from Central Asian countries like Uzbekistan, Turkmenistan and Tajikistan, local residents said on condition of anonymity out of fear for their safety. They also included some Arabs and fighters from Chechnya. Many foreign fighters in North Waziristan are closely allied with the Pakistani Taliban.

Pakistani Taliban militants were seen patrolling the area, but did not seem to be fleeing. Some local tribesmen were looking for homes outside of North Waziristan to which they could flee, but did not seem overly concerned about reports of an upcoming military operation.

By Friday, around 1,000 people, including wives and children of the foreign militants, had fled from four villages surrounding Mir Ali, one of the main towns in North Waziristan and a key sanctuary for militants fighting in Pakistan, said local residents. It appeared the militants would either head across the border to Afghanistan or to the neighboring South Waziristan tribal area.

The Pakistani army conducted a major offensive against the Pakistani Taliban in South Waziristan in 2009, but many militants simply fled and others still operate freely in certain areas.

Imtiaz Gul, director of the Islamabad-based Center for Research and Security Studies, said reports of an operation in North Waziristan would also give militants time to flee rather than face the army.

"They won't wait for the rockets to fall or the troops to capture them," said Gul.


Associated Press writers Zarar Khan and Rebecca Santana contributed to this report.