Palestinians visit the graves of their relatives who were killed in the war between Israel and the Hamas on the first day of Eid Al Fitr in Gaza on Wednesday. AP

The Israeli military said it was checking the report.

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.

Three of Haniyeh's grandchildren were also killed in the attack and a third was wounded, Hamas media said.

"The blood of my sons is not dearer than the blood of our people," Haniyeh, 61, who has 13 sons and daughters according to Hamas sources, told pan-Arab Al Jazeera TV.

The three sons and three grandchildren were making family visits during the first day of the Muslim Eid Al Fitr holiday in Shati, their home refugee camp in Gaza City, according to relatives.

Hamas said on Tuesday it was studying an Israeli ceasefire proposal in the more than six-month-old Gaza war but that it was "intransigent" and met none of the Palestinian demands.

"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 said.

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.

Reuters