The Britain-based rights group Amnesty International has called on the U.S. to end the secrecy surrounding its drone campaign in Pakistan.

In a report out Tuesday, Amnesty said the U.S. "appears to have committed very serious" human rights violations that might even amount to war crimes.

Amnesty's report outlines 45 missile strikes by pilotless planes in Pakistan's North Waziristan tribal territory between January 2012 to August 2013. It says in one strike, a 68-year-old grandmother was killed by a drone strike while out in the field picking vegetables. In another strike, it said, 18 laborers died by a drone strike as they prepared to eat their evening meal.

Pakistani leaders say they strongly oppose the drone strikes, but some critics believe the operations are part of a secret agreement between Pakistan and the United States in which Pakistan secretly approves the strikes.

Amnesty is calling on the United States and Pakistan to publicly disclose all information possible about the strikes. The report says locals in the area where the drones strike live in constant fear of violence from all sides.

U.N. Special Rapporteur Ben Emmerson has also called on the United States for more transparency. In preliminary findings last week, he quoted Pakistani officials as saying the drone attacks have killed at least 400 civilians.

U.S. authorities have offered little public information about the drone strikes but say they are carefully planned to avoid civilian casualties and have killed key al-Qaida operatives.