Christof Heyns, the UN special rapporteur on extrajudicial, summary or arbitrary executions, has said that certain types of drone attacks may constitute war crimes, says The Guardian. Heyns also expressed concern that the US's approach to drone strikes may encourage other states to flout well-established international norms in military engagement.
[Some states] find targeted killings immensely attractive. Others may do so in future. ... Killings may be lawful in an armed conflict but many targeted killings take place far from areas where it's recognized as being an armed conflict. [If] there have been secondary drone strikes on rescuers who are helping [the injured] after an initial drone attack, those further attacks are a war crime.