ISLAMABAD — U.S. drone strikes killed a Pakistani grandmother and 18 civilian laborers last year, Amnesty International said on Tuesday, Oct. 22, shedding new light on a major source of tension in U.S.-Pakistani relations.

Islamabad publicly opposes drone attacks, saying they kill too many civilians in addition to their intended target – Islamist militants. The precise extent of human loss on the ground is unclear, however, because independent journalists and researchers have only limited access to the affected regions.

Pakistan’s North Waziristan is the area of the most intensive U.S. drone campaign in the world. Many jihadi fighters have been eliminated, but neither the Pakistani government nor the United States releases details about those killed.

Amnesty said it had conducted a detailed investigation into two strikes in North Waziristan, yielding a report based on more than 60 interviews conducted by teams of researchers working independently of each other.

“We were really shocked, especially with the grandmother case. At first we thought, that can’t be true – there must be something more to this,” said Mustafa Qadri, the Amnesty researcher who wrote the report.

Demonstrators stand beside a mock drone outside the gates of Fort McNair where President Barack Obama will speak at Washington’s National Defense University May 23, 2013. Credit: Reuters/Kevin Lamarque

“People who are clearly no imminent threat to the U.S., are not fighting against the U.S., are being killed. The U.S. has to come clean publicly with the justifications for these killings.”

London-based Amnesty said a drone strike in the village of Ghundi Kala in October 2012 killed Mamana Bibi, 68, the wife of a retired school principal, as she was gathering vegetables.

Her five grandchildren were wounded, including Safdar, 3, who fell off a roof and broke bones in his chest and shoulders. It was unclear why the Bibi was hit. The weather was clear, providing good visibility to drone operators, the report said.

In the second incident, 18 men were killed in the village of Zowi Sidgi in July 2012. Residents described the dead as a woodcutter, vegetable seller and miners who had gathered in the shade at dusk to talk after a day’s work. The youngest was 14.

The first drone strike killed at least eight people in all, the report said. The second one killed more locals as they were trying to rescue the wounded.

“Everyone in the hut was cut to pieces,” Amnesty quoted one witness as saying. “We started to panic and each person was trying to run in a different direction.”

According to the London-based Bureau of Investigative Journalism, drones often also target rescuers coming to help those injured in an initial strike.

The Pakistani Taliban effectively control North Waziristan, and offer safe havens to al Qaeda and the Afghan Taliban who are fighting NATO troops across the border.

Residents told Amnesty that Afghan Taliban fighters often passed through the area and that some locals were supportive.

Pakistani authorities were not available to comment on the report. U.S. authorities have said they are not infringing on Pakistan’s territorial sovereignty as Pakistan has ceded to militants control of the area where most drone strikes occur.

The United States has carried out 376 drone strikes in Pakistan since 2004, the Bureau of Investigative Journalism says, with the death toll put at between 2,525 and 3,613. Local media reported that up to 926 of the dead were civilians.

Most of the time, the dead are militants although their rank is often unclear, residents, militants and Pakistani security sources have told Reuters. Government officials frequently say militant groups have killed 40,000 Pakistanis since 2001.