The UN human rights office on Friday charged that three men sentenced to death in Hamas-run Gaza were executed unlawfully.
UN High Commissioner for Human Rights Navi Pillay's spokesman, Rupert Colville, told reporters in Geneva that the death sentences carried out by hanging on April 7 weren't approved by Palestinian President Mahmoud Abbas as required by law.
He also said the men didn't have regular access to lawyers and were tried by a military court despite being civilians.
Pillay's office also called on Hamas to halt a planned execution by firing squad.
At least 18 men have been executed in Gaza since Hamas seized power in the coastal enclave in a bloody 2007 putsch.
However, human rights activists in Gaza say those figures only represent official executions, not the full range of human rights violations perpetrated by Hamas.
Hamas has routinely incarcerated, kidnapped, maimed and tortured dissidents and political rivals in Gaza over the past five years.
Notably, after Hamas seized control of Gaza, several members of the rival Fatah faction were dragged from their homes and shot in both legs.