ISLAMABAD, Pakistan -- A volley of American drone strikes in Pakistan's tribal belt early Friday killed at least 18 people, security officials said, marking a sharp escalation of the controversial C.I.A.-led campaign that continues to roil relations with Pakistan.
Drones fired at least six missiles at three locations in the Shawal Valley, a thickly-forested mountainous area in North Waziristan tribal agency, two Pakistani security officials and a local Taliban commander said.
The missiles struck mud-walled compounds and at least two vehicles, killing at least 18 people, one of the security officials said, speaking on the condition of anonymity. An official from the local administration confirmed the strikes.
The identity of those killed was not immediately known. Previous strikes in the area have targeted both individual commanders and fighters in trucks headed for the nearby Afghan border.
"It's a mixed bag," said a senior Pakistani security official, describing the militant groups in the Shawal Valley. "There are T.T.P. elements there and there are foreign militants there," he said referring to the Tehrik-i-Taliban Pakistan, commonly known as the Pakistani Taliban.
The attacks came one day after the Pakistani Foreign Office summoned an American Embassy official to protest earlier drone strikes in North Waziristan during the annual Id al-Fitr religious holiday, which ended on Wednesday.
It was the eighth time in 12 months that the Pakistani government had issued a formal protest on the issue, a senior Pakistani official said.
Although Pakistan and the United States patched up their differences over NATO supply routes in July, the drone strikes remain a major impediment to resuming normal relations between the two countries.
Pakistan's military has proposed that it can carry out the strikes itself using its fleet of American-built F-16 warplanes; human rights activists complain about civilian casualties and worry that the strikes further agitate anti-American hostility and could attract recruits for militant groups.
The Obama administration has rejected those demands, arguing that the covert campaign offers the most effective tool against Islamist militants hiding in an area where the Pakistani state has largely lost control.
North Waziristan, along the Afghan border, has borne the brunt of the drone campaign, which started in 2004. Friday was the fifth day of strikes in North Waziristan this month, four of which targeted the Shawal Valley.
Al Qaeda and Uzbek fighters fled to Shawal en masse from March 2002, after the United States military attacked their base in the mountains of southern Afghanistan.
The Pakistani military later moved in, setting up a major camp and building a road through the valley with the help of American aid. In 2005, Pakistani generals declared that the area had been purged of foreign militants; the latest C.I.A. strikes suggest that is no longer the case.
In recent weeks the Pakistani military has sought to quell rising speculation that it is planning a major operation in North Waziristan -- or at least, to quell the idea that such an operation was being dictated by the United States.
In an Independence Day speech on Aug. 14, the army chief, Gen. Ashfaq Parvez Kayani, emphasized the importance of fighting militancy at home -- a speech some interpreted as a harbinger for a coming operation. A senior American official said Pakistan had initially indicated that it would start its military drive after the Id al-Fitr holiday, but said it may now not take place until late September.
Following a major militant assault on Kamra air base, west of Islamabad, last week, Pakistan is seeking to pre-empt Islamist violence in the rest of the country.
The Interior Ministry temporarily shut down cellphone networks in several major cities over the Id al-Fitr holiday. The interior minister, Rehman Malik, said that militants frequently used fraudulently registered SIM chips to coordinate and execute attacks.
But a proposal by Mr. Malik to permanently ban prepaid mobile services -- used by about 95 percent of Pakistan's 120 million subscribers -- has met with a fierce backlash in the local media.
Relations between Pakistan and the United States are also tense over the Haqqani Network, a militant organization that operates from the Miram Shah area of North Waziristan, and which has blurry ties to Pakistani intelligence.
Under pressure from Congress, the Obama administration will have to decide by Sept. 9 whether it will declare the Haqqani Network a terrorist group, and risk worsening its relationship with Islamabad.
Ismail Khan contributed reporting from Peshawar, Ihsanullah Tipu Mehsud contributed reporting from Islamabad.
This article originally appeared in The New York Times.