Three sons of Hamas leader Ismail Haniyeh were killed in an Israeli airstrike in Gaza on Wednesday, the Palestinian Islamist group and Haniyeh's family said.

The three sons - Hazem, Amir and Mohammad - were killed after the car they were driving in was bombed in Gaza's Al-Shati camp, Hamas said. Two of Haniyeh's grandchildren were also killed in the attack and a third was wounded, Hamas media said.

"Our demands are clear and specific and we will not make concessions on them. The enemy will be delusional if it thinks that targeting my sons, at the climax of the negotiations and before the movement sends its response, will push Hamas to change its position," Haniyeh told pan-Arab Al Jazeera TV.

"The blood of my sons is not dearer than the blood of our people," said Haniyeh, who is based abroad in the Gulf Arab state of Qatar.

Haniyeh has been the tough-talking face of Hamas' international diplomacy as war with Israel has raged back in the Gaza Strip, where his family home was destroyed in an Israeli airstrike back in November.

Hamas said on Tuesday it was studying an Israeli ceasefire proposal but that it was "intransigent" and did not meet any of the Palestinian demands.

In the seventh month of a war in which Israel's air and ground offensive has devastated Gaza, Hamas wants an end to Israeli military operations and a withdrawal from the enclave, and permission for displaced Palestinians to return home.

Haniyeh's eldest son confirmed in a Facebook post that his three brothers were killed. "Thanks to God who honoured us by the martyrdom of my brothers, Hazem, Amir and Mohammad and their children," wrote Abdel-Salam Haniyeh.

Appointed to the militant group's top job in 2017, Haniyeh has moved between Turkey and Qatar's capital Doha, avoiding Israeli-imposed travel restrictions in blockaded Gaza and enabling him to act as a negotiator in the latest ceasefire negotiations, or communicate with Hamas' main ally Iran.

Israel regards the entire Hamas leadership as terrorists, accusing Haniyeh and other leaders of continuing to "pull the strings of the Hamas terror organisation".

But how much Haniyeh knew about the Oct. 7 cross-border attack on Israel by Gaza-based militants beforehand is not clear. The attack plan, drawn up by the Hamas military council in Gaza, was such a closely guarded secret that some Hamas officials abroad seemed shocked by its timing and scale.