The CIA's programme of "targeted" drone killings in Pakistan's tribal heartlands is politically counterproductive, kills large numbers of civilians and undermines respect for international law, according to a report by US academics.

The study by Stanford and New York universities' law schools, based on interviews with victims, witnesses and experts, blames the US president, Barack Obama, for the escalation of "signature strikes" in which groups are selected merely through remote "pattern of life" analysis.

Families are afraid to attend weddings or funerals, it says, in case US ground operators guiding drones misinterpret them as gatherings of Taliban or al-Qaida militants.

"The dominant narrative about the use of drones in Pakistan is of a surgically precise and effective tool that makes the US safer by enabling 'targeted killings' of terrorists, with minimal downsides or collateral impacts. This narrative is false," the report, entitled Living Under Drones, states.

The authors admit it is difficult to obtain accurate data on casualties "because of US efforts to shield the drone programme from democratic accountability, compounded by obstacles to independent investigation of strikes in North Waziristan".

The "best available information", they say, is that between 2,562 and 3,325 people have been killed in Pakistan between June 2004 and mid-September this year – of whom between 474 and 881 were civilians, including 176 children. The figures have been assembled by the Bureau of Investigative Journalism, which estimated that a further 1,300 individuals were injured in drone strikes over that period.

The report was commissioned by and written with the help of the London-based Reprieve organisation, which is supporting action in the British courts by Noor Khan, a Pakistani whose father was killed by a US drone strike in March 2011. His legal challenge alleges the UK is complicit in US drone strikes because GCHQ, the eavesdropping agency, shares intelligence with the CIA on targets for drone strikes.

"US drones hover 24 hours a day over communities in north-west Pakistan, striking homes, vehicles, and public spaces without warning," the American law schools report says.

"Their presence terrorises men, women, and children, giving rise to anxiety and psychological trauma among civilian communities. Those living under drones have to face the constant worry that a deadly strike may be fired at any moment, and the knowledge that they are powerless to protect themselves.

"These fears have affected behaviour. The US practice of striking one area multiple times, and evidence that it has killed rescuers, makes both community members and humanitarian workers afraid or unwilling to assist injured victims."

The study goes on to say: "Publicly available evidence that the strikes have made the US safer overall is ambiguous at best … The number of 'high-level' militants killed as a percentage of total casualties is extremely low – estimated at just 2% [of deaths]. Evidence suggests that US strikes have facilitated recruitment to violent non-state armed groups, and motivated further violent attacks … One major study shows that 74% of Pakistanis now consider the US an enemy."

Coming from American lawyers rather than overseas human rights groups, the criticisms are likely to be more influential in US domestic debates over the legality of drone warfare.

"US targeted killings and drone strike practices undermine respect for the rule of law and international legal protections and may set dangerous precedents," the report says, questioning whether Pakistan has given consent for the attacks.

"The US government's failure to ensure basic transparency and accountability in its targeted killings policies, to provide details about its targeted killing programme, or adequately to set out the legal factors involved in decisions to strike hinders necessary democratic debate about a key aspect of US foreign and national security policy.

"US practices may also facilitate recourse to lethal force around the globe by establishing dangerous precedents for other governments. As drone manufacturers and officials successfully reduce export control barriers, and as more countries develop lethal drone technologies, these risks increase."

The report supports the call by Ben Emmerson QC, the UN's special rapporteur on countering terrorism, for independent investigations into deaths from drone strikes and demands the release of the US department of justice memorandums outlining the legal basis for US targeted killings in Pakistan.

The report highlights the switch from the former president George W Bush's practice of targeting high-profile al-Qaida personalities to the reliance, under Obama's administration, of analysing patterns of life on the ground to select targets.

"According to US authorities, these strikes target 'groups of men who bear certain signatures, or defining characteristics associated with terrorist activity, but whose identities aren't known'," the report says. "Just what those 'defining characteristics' are has never been made public." People in North Waziristan are now afraid to attend funerals or other gatherings, it suggests.

Fears that US agents pay informers to attach electronic tags to the homes of suspected militants in Pakistan haunt the tribal districts, according to the study. "[In] Waziristan … residents are gripped by rumours that paid CIA informants have been planting tiny silicon-chip homing devices that draw the drones.

"Many of the Waziris interviewed spoke of a constant fear of being tagged with a chip by a neighbour or someone else who works for either Pakistan or the US, and of the fear of being falsely accused of spying by local Taliban."

Reprieve's director, Clive Stafford Smith, said: "An entire region is being terrorised by the constant threat of death from the skies. Their way of life is collapsing: kids are too terrified to go to school, adults are afraid to attend weddings, funerals, business meetings, or anything that involves gathering in groups.

"George Bush wanted to create a global 'war on terror' without borders, but it has taken Obama's drone war to achieve his dream."