In an interview with CNN, Erakat labeled the attack the beginning of a "major, major Israeli escalation," and he called on the international community to pressure Israel to halt its operation.
Al-Mukhalalati, the Gaza health minister, also called on "the free world to stop this massacre committed by Israel."
Asked about assassinating al-Ja'abari, Regev said the Hamas military leader headed a "terror military machine."
"This is the man with blood on his hands. This man is a known and wanted terrorist," he said. "In taking him out, Israel was acting legitimately."
The latest escalation in violence is part of a cycle of attacks between periods of relative stability between Israel and the Palestinians.
"This was both inevitable and predictable," said Aaron David Miller, a Middle East expert and vice president at the Woodrow Wilson Center.
There is no long-term basis for both sides to find a lasting peace, he said.
Israel and the Palestinians have "completely different political and strategic goals," Miller explained. The focus should be on how to bring security until a basis for long-term stability arises, he added.