(CBS/AP> ISLAMABAD American missiles slammed into militant hideouts close to the Afghan border Friday, killing 18 suspected militants, Pakistani officials said, just a day after the government summoned an American diplomat to protest the drone strikes in the tribal areas.
The strikes Friday were the fourth attack in the span of a week, as well as the most deadly.
The frequency of such attacks appears to suggest an escalation of the efforts, CBS News experts say.
The drone campaign has been a source of friction between the U.S. and Pakistan, which sees the strikes as an infringement on its sovereignty. The U.S. maintains the campaign is vital to combating militants, including al Qaeda, operating in Pakistan's northwest tribal region near the Afghan border.
On Thursday, Pakistan's Foreign Ministry summoned a U.S. diplomat to protest the recent drone strikes.
"A senior U.S. diplomat was called to the Ministry of Foreign Affairs and informed that the drone strikes were unlawful, against international law and a violation of Pakistan's sovereignty. It was emphatically stated that such attacks were unacceptable," the Foreign Ministry said in a statement. The diplomat was not identified.
One day later, Pakistani intelligence officials said American drone-fired missiles hit three militant hideouts in Pakistan's North Waziristan tribal areas.
The officials said each of the three compounds was hit by two missiles. Militants often use these hideouts when they are crossing into Afghanistan, the officials said.
Fourteen people were also injured in the attack.
The officials spoke on condition of anonymity because they were not authorized to speak to reporters. There was no immediate U.S. comment.
Islamabad's protest followed a string of three drone attacks earlier this week.
On Saturday, five allies of a powerful warlord, Hafiz Gul Bahadur, whose forces often strike U.S. troops in Afghanistan, died when a U.S. drone struck their hideout. On Sunday American drones fired a flurry of missiles into the Pakistani tribal area bordering Afghanistan, killing 10 suspected militants. On Tuesday, missiles targeting a vehicle killed five more suspected militants.
All the strikes this week came in North Waziristan, one of the last areas of the tribal region in which the Pakistani military has not conducted any operations against militants. The U.S. has pushed repeatedly for Pakistan to open an offensive there, and U.S. Secretary of Defense Leon Panetta recently said Pakistani authorities would start a campaign there soon. So far there are few signs on the ground of a large-scale offensive.
The drone strikes are unpopular in Pakistan because many people believe they mostly kill civilians an allegation disputed by the U.S.
Despite Pakistan's public protests, the government is widely believed to have supported the attacks quietly in the past. That cooperation has come under pressure as the relationship between the two countries has deteriorated.
The U.S. shows no sign that it is willing to end or curtail the contentious program.