Navi Pillay said she was "concerned about the process by which death sentences are imposed by military and civilian courts in the Gaza Strip," the U.N. Commission for Human Rights said in a statement.
International human rights law requires "that the death penalty can only be imposed after a fair trial," Pillay added. "This is currently not possible in Gaza, neither legally nor practically."
She added serious concerns had been raised about "ill-treatment and torture during interrogations of persons later sentenced to death."
Gaza's attorney general, Ismail Jaber, announced last week that executions of some convicted criminals would begin after the end of Ramadan, which concluded Aug. 7, Al-Monitor reported.
In an Aug. 1 statement, Jaber said the executions would begin "immediately after the [Eid al-Fitr] holiday, adding the executions would be "public, in retaliation for those who died."
He repeated his statement Sunday, saying the executions were awaiting the approval of the Council of Ministers.
Gaza's Prime Minister Ismail Haniyeh confirmed earlier in the month the executions would be carried out. While Haniyeh said he respected local human rights organization calling for the executions to be canceled, "we also respect our blood, our Sharia and our law."