In Yemen, Human Rights Watch investigated six airstrikes since 2009 and found at least 57 of the 82 people killed were civilians, The Washington Post reported Tuesday.
In Pakistan, Amnesty International said it investigated nine suspected U.S. drone strikes from May 2012 to July 2013 in North Waziristan, finding strong evidence that at least 30 civilians were killed in four of the attacks.
While the circumstances of the drone strikes were reported by media, the human rights groups said they found more information on the incidents by interviewing survivors, witnesses and government officials.
The two studies were to be released Tuesday, The New York Times reported. On Wednesday, Pakistani Prime Minister Nawaz Sharif, a critic of the drone campaign, is scheduled to meet with President Obama at the White House. On Friday, a drone debate is scheduled at the United Nations, whose human rights investigator recently published a report critical of the United States' lack of transparency over drones.
Both Human Rights Watch and Amnesty International called on the Obama administration for more transparency in its drone operation policies and to investigate reports of civilian casualties publicly.
"The full picture will only come to light when U.S. authorities fully disclose the facts, circumstances and legal basis for each of its drone strikes," Amnesty International said in its report.
White House spokeswoman Caitlin Hayden declined to comment to the Post on the reports.
However, she cited a speech President Obama gave in May in which he announced narrower guidelines for drone attacks. In the speech, Obama said unmanned aircraft would be used only against people posing a "continuing, imminent threat" to the United States and only when avoiding civilian casualties would be "a near-certainty."
"As the president emphasized, the use of lethal force, including from remotely piloted aircraft, commands the highest level of attention and care," Hayden said in an e-mail to the Post.
The CIA carries out drone strikes in Pakistan under a covert program. In Yemen, the CIA and the Defense Department's Joint Special Operations Command conduct drone attacks. Representatives for the CIA and the Pentagon declined to comment, the Post said.
The number of U.S. drone strikes in Pakistan and Yemen fell during the past year, officials said.