The decision revealed by two senior Hamas officials on Saturday would help a bid led by the Palestinian president, Mahmoud Abbas, to join the court, a step that would transform his relations with Israel from tense to openly hostile and could also strain his ties with the United States.
Abbas has said he will not make any decision on a bid without the written backing of all Palestinian factions. Last month, he obtained such support from all factions in the Palestine Liberation Organisation.
The move by Hamas, which is not a PLO member, came after almost seven weeks of a cross-border war with Israel and several failed ceasefire efforts.
More than 2,090 Palestinians have been killed since fighting began on 8 July, including around 500 children, and about 100,000 Gazans have been left homeless, according to United Nations figures and Palestinian officials. Israel has lost 64 soldiers and four civilians, including a four-year-old boy killed by a mortar shell on Friday.
During the war, Gaza militants have fired more than 3,800 rockets and mortar shells at Israel, while Israel has launched about 5,000 air strikes at Gaza, the military said. Israel has said it has targeted sites linked to militants. UN and Palestinian officials say three-quarters of those killed in Gaza have been civilians.
On Saturday, an air strike on a house in central Gaza killed two women, two children and a man, according to medics at the Red Crescent. Six strikes also hit a house in the Zeitoun neighbourhood of Gaza, causing severe damage and wounding at least five people, Gaza police said.
Since the start of the Gaza war, Abbas has come under growing domestic pressure to pave the way for a possible war crimes investigation of Israel. Last month, he told senior PLO officials and leaders of smaller political groups he would only go ahead if Hamas supported the bid.
If Abbas were to turn to the court, Hamas could be investigated for indiscriminate rocket fire at Israel since 2000. Israel could come under scrutiny for its actions in the current Gaza war as well as decades of settlement building on war-won lands the Palestinians seek for a state.
Izzat Rishq, a senior Hamas official, said on Saturday that Hamas was not concerned about becoming a target of a war crimes investigation and urged Abbas to act “as soon as possible”.
“We are under occupation, under daily attack and our fighters are defending their people,” he said in a phone interview from Qatar. “These rockets are meant to stop Israeli attacks and it is well known that Israel initiated this war and previous wars.”
But it is uncertain whether such arguments would hold up in court. After the last major round of Israel-Hamas fighting more than five years ago, a UN fact-finding team said both Israel and Hamas violated the rules of war by targeting civilians.
The Hamas decision to back a court bid came after meetings on Thursday and Friday in Qatar between Abbas and the top Hamas leader in exile, Khaled Mashaal.
Moussa Abu Marzouk, a senior Hamas leader who participated in the meetings, wrote on his Facebook page early on Saturday that “Hamas has signed the paper” of support that Abbas had requested. Abu Marzouk’s post was also reported on Hamas news websites.
There was no comment from Abbas aides.
A senior Palestinian official has said Abbas was expected to wait for the findings of a UN-appointed commission of inquiry into possible Gaza war crimes due by March before turning to the court.
The official spoke on condition of anonymity because he was not authorized to discuss internal deliberations with reporters.
The office of Israeli prime minister, BinyaminNetanyahu, declined to comment. Israel opposes involving the court, arguing that Israel and the Palestinians should deal with any issues directly.