Human rights lawyers have warned that alleged illegal drone attacks by the US in Pakistan and Yemen risk encouraging their use by other states in the region.

Andrea Prasow, a counter-terrorism lawyer with Human Rights Watch, said a new report with Amnesty International into evidence of strikes against civilians showed breaches of international conventions that could be exploited by hostile countries using similar technology.

"The failure to abide by international law sets a dangerous precedent for other countries," Prasow told reporters at the launch of the report.

On Monday, Iranian media reported that authorities in Tehran had reverse-engineered a captured US drone and made a series of copies.

Naureen Shah of Amnesty echoed the growing concern in legal circles that indiscriminate US strikes will make it harder to restrain such countries in future.

"Drone technology is proliferating rapidly. The US government should be careful of the signal that it is sending to the world," she said. "It's time to end the black hole of accountability on drone strikes. The US is behaving like a hit-and-run driver."

Though it disputes the casualty estimates and legal claims of non-governmental organisations, even the US administration admits it is worried about the spread of drone warfare. "Going forward this is a technology that we know more people will probably get access to," State Department spokeswoman Marie Harf told the Guardian.

"This is not just about when we decide to use this technology but when other countries do to. This is at the forefront of our counterterrorism officials' minds and something we will certainly be discussing."

The State Department also said it would consider other aspects of the two reports in more details but was dismissive about their ability to accurately assess casualty numbers, and insists civilian deaths are "much lower" than NGO estimates.

"Our intelligence community has ways to get information across the board that gives a much more complete picture than one or two groups can get from talking to folks on the ground," said Harf.

A house in Pakistan near the biorderd with Afghanistan destroyed by a drone misile in 2008. Eighteen people including foreign militants were killed. Photograph: Stringer Pakistan / Reuters/Reuters

Getting to the bottom of individual strikes is exceptionally difficult in the restive areas bordering Afghanistan, where thousands of militants have settled. People are often terrified of speaking out, fearing retribution from both militants and the state, which is widely suspected of colluding with the CIA-led campaign.

There is also a risk of militants attempting to skew outside research by forcing interviewees into "providing false or inaccurate information", the report said.

But Amnesty and Human Rights Watch say they interviewed a total of 150 witnesses and survivors in Yemen and Pakistan during extensive visits to the scene of US attacks, and cross-checked all their reports with local authorities.

For its report, Amnesty mounted a major effort to investigate nine of the many attacks to have struck Pakistan over the last 18 months, including one that killed 18 labourers in North Waziristan as they waited to eat dinner in an area of heavy Taliban influence in July 2012. All those interviewed by Amnesty strongly denied any of the men had been involved in militancy. Even if they were members of a banned group, that would not be enough to justify killing them, the report said.

"Amnesty International has serious concerns that this attack violated the prohibition of the arbitrary deprivation of life and may constitute war crimes or extrajudicial executions," the report said. It called for those responsible to stand trial.

The US has repeatedly claimed very few civilians have been killed by drones, and argues that its campaign is lawful. At the State Department, Harf insisted the US "took every effort to minimise civilian casualties in counter-terrorism operations" and said drones were "often the option least likely to result in the loss of civilian lives".

People gather at the site of a drone strike on the road between Yafe and Radfan districts of the southern Yemeni province of Lahj in August 2013. Photograph: Reuters

The Amnesty report supports media accounts from October last year that a 68-year-old woman called Mamana Bibi was killed by a missile fired from a drone while she was picking okra outside her home in North Waziristan with her grandchildren nearby. A second strike minutes later injured family members tending her.

If true, the case is striking failure of a technology much vaunted for its accuracy. It is claimed the remote-controlled planes are able to observe their targets for hours or even days to verify them, and that the explosive force of the missiles is designed to limit collateral damage. As with other controversial drone strikes, the US has refused to acknowledge or explain what happened.

Amnesty said it accepts some US drone strikes may not violate the law, "but it is impossible to reach any firm assessment without a full disclosure of the facts surrounding individual attacks and their legal basis. The USA appears to be exploiting the lawless and remote nature of the region to evade accountability for its violations," it said.

In Yemen, another country where US drones are active, Human Rights Watch highlighted six incidents, two of which were a "clear violation of international humanitarian law". The remaining four may have broken the laws of armed conflict because the targets were illegitimate or because not enough was done to minimise civilian harm, the report said.

It also argued that some of the Yemen attacks breach the guidelines announced by Obama earlier this year in his first major speech on a programme that is officially top secret. For example, the pledge to kill suspects only when it is impossible to capture them appears to have been ignored on 17 April this year when an al-Qaida leader was blown up in a township in Dhamar province in central Yemen, Human Rights Watch said.

An attack on a truck driving 12 miles south of the capital Sana'a reportedly killed two al-Qaida suspects but also two civilians who had been hired by the other men. That means the attack could have been illegal because it "may have caused disproportionate harm to civilians".

At the press conference in Washington, Prasow, the Human Rights Watch lawyer, said the sentiments expressed by Obama in his speech had yet to be put into practice. "President Obama's speech in July was a big step forward but it was a speech detailing policy not law and has not yet been followed by greater transparency," Prasow said. "The US still has secret rules to kill its enemies and a secret list of those enemies."

Letta Taylor of Human Rights Watch added: "Drone strikes have ebbed and flowed in the past. It is too early to tell if there is a real change in US policy but it is important to set a benchmark for future administrations too."