VIDEO: What is a 'double-tap' strike?

"Double-tap" strikes are becoming increasingly common in modern warfare.

The controversial military tactic is defined by armed drones or war planes attacking a site and then returning to attack the same area again as people are carrying out rescue work.

Pro-government forces in Syria, the Saudi-led colaition in Yemen, and Israeli operations in Gaza have all come under fire for reportedly intentionally targeting civilians and first-responders attending to the injured.

Christof Heyns, the former UN special rapporteur on extrajudicial killings told a meeting in Geneva in 2012 that "if civilian ‘rescuers’ are indeed being intentionally targeted, there is no doubt about the law: those strikes are a war crime."

The Saudi-led coalition is facing international outcry after an alleged "double-tap" air strike killed 140 people and left 534 people wounded in Yemen.