Palestinian resistance group Hamas on Wednesday vowed a strong response to the assassination of its political bureau chief Ismail Haniyeh.

A senior Hamas official said Wednesday that Haniyeh's killing "will not go unanswered."

"The assassination of leader Ismail Haniyeh is a cowardly act and will not go unanswered," Hamas policial bureau member Musa Abu Marzuk said in a statement.

Meanwhile, Palestinian president Mahmud Abbas also condemned the killing of Haniyeh, calling it a "cowardly act" and urged Palestinians to remain united against Israel.

"President Mahmud Abbas of the State of Palestine strongly condemned the assassination of Hamas leader Ismail Haniyeh, deeming it a cowardly act and a serious escalation," Abbas's office said in a statement.

"He urged our people and their forces to unite, remain patient, and stand firm against the Israeli occupation."

Another Palestinian resistance group Islamic Jihad described the killing of Haniyeh as an "abominable murder."

In a statement, the group said Haniyeh was a "symbol of resistance" and a great leader. They asserted that his death would not deter the Palestinian people from their ongoing resistance.

Ismail Haniyeh's son Abdul Salam Haniyeh, in his reaction said: "My father survived four assassination attempts during his patriotic journey, and today Allah has granted him the martyrdom that he always wished for.

"He was very keen to establish national unity and strived for the unity of all Palestinian factions and we affirm that this assassination will not deter the resistance, which will fight until freedom is achieved."