A US air raid has killed 18 suspected fighters near Pakistan's border with Afghanistan, Pakistani officials have said.

Fourteen other people were also reportedly injured after drone-fired missiles hit three compounds used by the fighters in North Waziristan on Friday, a day after Islamabad summoned a US diplomat to protest a recent series of strikes that have caused much collateral damage and civilian casualties.

The raid was the fourth attack in the span of a week, as well as the most deadly. 

Officials, on condition of anonymity, said each of the three compounds, which are often used hideouts for fighters when they cross into Afghanistan, was hit by two missiles.

The drone campaign, which Washington sees as vital to combating armed groups, including al-Qaeda, has been a cause of friction between the two countries, as Pakistan sees the strikes as an infringement on its sovereignty. 

"A senior US 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 Pakistani foreign ministry said in a statement. The diplomat was not identified.

Last week, five allies of a powerful warlord, Hafiz Gul Bahadur, whose forces often strike US troops in Afghanistan, died when a US drone struck their hideout in North Waziristan. 

Pakistan campaign

On Sunday, US drones fired a flurry of missiles into the Pakistani tribal area bordering Afghanistan, killing 10 suspected fighters. On Tuesday, missiles targeting a vehicle killed five more suspected fighters.

All the strikes this week occurred in North Waziristan, one of the last areas of the tribal region in which the Pakistani military has not conducted any operations against fighters.

The US has pushed repeatedly for Pakistan to open an offensive there, and US 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 US. 

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 US shows no sign that it is willing to end or curtail the controversial usage of drones.