US officials responsible for the secret CIA drone campaign [in] Pakistan may have committed war crimes and should stand trial, a report by a leading human rights group warns. Amnesty International has highlighted the case of a grandmother who was killed while she was picking vegetables and other incidents which could have broken international laws designed to protect civilians. The report is issued in conjunction with an investigation by Human Rights Watch detailing missile attacks in Yemen which the group believes could contravene the laws of armed conflict, international human rights law and Barack Obama's own guidelines on drones. 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. But Amnesty mounted a major effort to investigate nine of the many attacks to have struck the region 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. 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.