Rights groups have demanded that the US launch an impartial investigation into its use of drone warfare and that the country publicly disclose any evidence of civilian casualties.
In independent reports published on Tuesday, both Amnesty International and Human Rights Watch said that the US must hold to account those responsible for civilian deaths and be more transparent about its drone programme.
"As evidence emerges of civilian casualties in these strikes, it’s time for the US to stop covering its ears and starting taking action to ensure the programme is legal," Letta Taylor, senior counterterrorism researcher at HRW told Al Jazeera.
Two recently published UN reports are to be presented to the General Assembly on Friday. Taylor said that the release of the four reports in a brief period "underscores the mounting questions about the legality" of drones.
All four reports demand that the US should provide a full legal rationale for targeted killings.
Polly Truscott, Amnesty International’s deputy Asia-Pacific director, said that while its report focuses on Pakistan, and HRW’s on Yemen, the drones programme "raises the same questions about human rights violations all over the world".
"Both organisations are calling on the US Congress to fully investigate the cases the we have documented in our reports and other potentially unlawful deaths," she told Al Jazeera, adding that the group hoped that the US would act immediately on their recommendations.
HRW said that the Yemeni government, which is engaged in a conflict with al-Qaeda, had been "almost as silent" as the US on the death toll caused by air raids.
Caitlin Hayden, a spokesman for the US National Security Council, said that President Barack Obama had outlined the US rational for drone strikes in a May 23 speech.
"The president spoke at length about the policy and legal rationale for how the US takes action against al-Qaeda and its associated forces. As the president emphasised, the use of lethal force, including from remotely piloted aircraft, commands the highest level of attention and care.
"Of particular note, before we take any counterterrorism strike, there must be near-certainty that no civilians will be killed or injured - the highest standard we can set."
She said the US was aware that this report had been released and were reviewing it carefully.
Through personal testimonies from witnesses and relatives of drone-strike victims in Pakistan's North Waziristan, Amnesty International's 74-page report titled "Will I be next? US drone strikes in Pakistan" reviewed 45 known drone strikes in the region between January 2012 and August this year.
It found that nine of the strikes could amount to war crimes or extrajudicial executions - some of which were unjustified and some of which were cases of "rescuer" or follow-up attacks on residents who had gone to the scene after an initial strike.
The bodies were charred like coal - I could not recognize the faces
In July 2012, 18 labourers, including a 14-year-old boy, were killed in multiple strikes on a village close to the Afghan border. The report indicates that the victims of the strike were not involved in fighting and did not pose a threat.
The London-based rights group said such strikes were caused locals to live in fear, and set "a dangerous precedent that other states may seek to exploit to avoid responsibility for their own unlawful killings".
HRW’s 97-page report, Between a Drone and al-Qaeda': The Civilian Cost of US Targeted Killings in Yemen, examined six US targeted killings in the country - one from 2009, and the rest between 2012 and 2013.
The strikes killed 82 people, at least 57 of them civilians.
None met US policy guidelines for targeted killings set out in US President Barack Obama's speech in May, said the New York-based rights group.
"Two of the attacks killed civilians indiscriminately in clear violation of the laws of war; the others may have targeted people who were not legitimate military objectives or caused disproportionate civilian deaths," said HRW.
A witness quoted in the report described the aftermath of one strike targeting an alleged al-Qaeda leader, but instead struck a passenger van killing 12 civilians.
"The bodies were charred like coal - I could not recognise the faces," Ahmad al-Sabooli, a 23 year-old Yemeni farmer, said.
International law prohibits arbitrary killings and limits intentional lethal force to exceptional situations wherein in an armed conflict, only combatants and those participating in hostilities may be targeted.
Intentional lethal force is lawful only when there is, with certainty, an imminent threat to life.