Their stories flow like water.

One lost a son and brother, killed by a missile that tore from the sky as they sat talking with others; one man’s father was among those who died when a missile hit a tribal gathering. Another man, a wage labourer, was in the local bazaar when he heard his father was also among the dozens killed in the same incident.

These individuals and their stories represent the collateral damage of America’s drone missile programme in Pakistan, a covert CIA operation that has been conducted largely out of sight and beyond scrutiny.

Earlier this year, in his first public comment on the programme, President Barack Obama claimed the operations involved “precision strikes” against anti-American targets. “A lot of these strikes have been in [Pakistan’s tribal areas],” he said. “For the most part, they’ve been very precise precision strikes against al-Qa’ida and their affiliates, and we’re very careful in terms of how it’s been applied.”

But increasingly, campaigners are pointing out the civilian cost of the operations, which are deeply unpopular and a source of widespread anti-Americanism despite the suspicion that Pakistan’s military co-operates. They say that, while the US insists only “suspected militants” are targeted, there is mounting evidence ordinary people are among the victims. “There has to be some transparency, some rules and regulations,” said Mirza Shahzad Akbar, who heads the Foundation for Fundamental Rights, an Islamabad-based NGO. “I don’t believe the US is going to give up its drones worldwide…but the CIA is acting on its own, without any accountability. For an educated Pakistani such as myself, who speaks out against extremism, it’s very hard when you see the example set by the developed world.”

Last weekend, Mr Akbar’s organisation brought 300 people from North Waziristan to Islamabad to protest. Between 70-80 were related to victims, the others were tribal elders. Many held photographs of those they had lost.

Among them was Kareem Khan, a 50-year-old from Machikhel, North Waziristan, who works as a journalist with an “Arab TV channel”. His 18-year-old son and brother were killed when a drone missile struck a community gathering on New Year’s Eve 2009. “They were both government employees – my brother was an English teacher at a government school and had done his Masters in English,” said Mr Khan. “And my son, who had completed his high schooling from Islamabad, he was a guard at a girls’ school. They were both martyred.”

Mr Khan became increasingly distressed and added: “A human is human. If one loses someone close, one weeps. Our hearts are still of flesh. When someone loses their son or brother or another relative they become depressed. My wife too, is very sad.”

One of the most notorious strikes took place on 17 March last year at Datta Khel, North Waziristan, when a missile hit a meeting, or jirga, called to resolve a dispute between chromite-mining contractors. Up to 50 people died. The US claimed a large number of militants attended the meeting but researchers have found several dozen civilians were also killed. Noor Khan, 26, who attended the demonstration, said his father, a council leader, Malik Daud Khan Mada Khel, was among the civilians who perished.

“It was a tragedy, not just [for] me but the whole family, because he was our community leader and he dealt with our problems in a peaceful way,” said the man’s son. “He was a tribal leader, so he was widely respected because he solved their problems.”

Imran Khan Wazir, a day labourer, said his father, Malik Ismail, was also killed at the jirga. He said he was in the market in Miran Shah, the main town in North Waziristan, when he learned the news. “It’s been a huge loss. I’m the only brother, and now I have the responsibility of looking after the family,” he said. “America – stop these attacks. These are wicked acts. Many innocents are dying. Our jirga was innocent.”

There is no agreed figure for how many people have been killed by the strikes or what percentage of them may have been genuine militants. The New America Foundation, a Washington think tank, says over the years 2004-2012 between 1,741 to 2,712 people were killed, of which an estimated 1,448 to 2,241 were militants.

The Bureau For Investigative Journalism believes that between 2,383 and 3,109 people have been killed, of whom 464 to 815 were civilians, which would mean the percentage of militants killed was 70 to 80 percent. The Associated Press this week said its own investigation of 10 drone strikes had found that of 194 people killed, about 70 per cent were militants.

Mr Akbar said one of the problems in assessing who had been killed was that many tallies were based on media reports which relied for information upon either US or Pakistani intelligence officials, or else so-called political agents, administrative officials in the tribal areas. “What do they mean when they say someone ‘acted in a manner consistent with al-Qa’ida militants’,” said Mr Akbar. “Do they mean they had a beard, do they mean they had a gun? Because almost everyone in the tribal areas is carrying a gun, but it doesn’t mean they are a militant.”

Inside the US, the debate continues as to whether the drone programme is legal. In a speech last week at the Yale Law School, Jeh Charles Johnson, legal advisor to the Pentagon, said operations against groups such as al-Qa’ida were covered by the Authorization for the Use of Military Force legislation passed by the US Congress a week after the attacks of 9/11. He went on to defend “targeted killing” by saying: “In an armed conflict, lethal force against known, individual members of the enemy is a long-standing and long-legal practice.”

Not everyone agrees. Writing in the Jurist, Gabor Rona, international legal director for Human Rights First, said such rules may have worked for conflicts between states, when those involved could be identified. “In armed conflicts against non-state armed groups who do not wear uniforms and are often difficult to distinguish from the civilian population, targeting determinations rightfully require a higher threshold of imminent harm,” he wrote.