(CNN) -- Two international human rights groups have raised grave concerns about U.S. drone strikes in Pakistan and Yemen, suggesting some attacks in recent years may amount to war crimes.
Released Tuesday, the highly critical reports by Amnesty International and Human Rights Watch give detailed accounts of a number of attacks they say the United States carried out in each of the two countries, some of which killed civilians.
Amnesty International said in its report on drone attacks in northwest Pakistan that it is "seriously concerned" that some of the strikes "have resulted in unlawful killings that may constitute extrajudicial executions or war crimes."
Two attacks its report highlights include the killing of a 68-year-old grandmother and the deaths of 18 laborers.
Human Rights Watch said two of the six attacks in Yemen it investigated "were in clear violation of international humanitarian law -- the laws of war," while the other four "may have violated the laws of war."
Both groups called for impartial investigations into the cases they have documented in their reports, as well as a series of other measures to comply with international law.
And they underlined the difficulties of researching attacks in dangerous areas of Pakistan and Yemen, noting the U.S. government's unwillingness to talk about the cases.
CNN was unable to reach U.S. officials for comment early Thursday.
The U.S. government has said strikes by the unmanned aircraft are a necessary part of the fight against militant groups. In May, President Barack Obama defended the use of the drone program.
The release of the Amnesty report comes the day before Pakistani Prime Minister Nawaz Sharif is due to meet Obama in Washington.
The campaign of drone strikes have stirred deep anger in Pakistan, and Sharif has called for it to end.