Two young girls were killed in northern Gaza when their house was hit by a mortar round apparently fired by Palestinian militants targeting southern Israel.
Palestinian medics say a mortar shell intended for Israel instead hit a house in Beit Lahya, killing the girls, who were aged 12 and five. Four other family members were wounded.
The Israeli military says its forces haven't fired in that area of the Gaza Strip.
The casualties have occurred amid mounting speculation that Israel will soon launch a ground/air operation in the coastal enclave. Violence in and around Gaza has flared since a six-month ceasefire ended on 19 December, and it escalated dramatically on 24 December when Israel launched deadly air strikes and militants fired back.
At least six militants have been killed by the air strikes and dozens of rockets and mortar shells from Gaza have slammed into Israel, damaging homes and causing panic.
Israel had responded to earlier rocket attacks by tightening the blockade it imposed on Gaza in June 2007, after the militant Hamas movement took power there.
But officials says dozens of truckloads of supplies were delivered to Gaza on Friday after Israel temporarily allowed humanitarian aid in - responding, it said, to numerous requests from the international community.
Palestinian workers at the border crossings said fuel had arrived for Gaza's main power plant. Shortages have caused periodic blackouts for many of the territory's 1.5 million residents.
Major offensive threatened
Since the ceasefire ended, Israel has threatened to launch a major offensive on Gaza, and senior leaders have called for the toppling of Hamas.
The Islamist movement - which is sworn to the destruction of Israel - has warned in turn that it would retaliate by resuming suicide bombings inside Israel.
Meanwhile, Iran's Red Crescent is sending 2000 tonnes of food, medicine and appliances as well as 12 doctors and relief workers to Gaza in defiance of Israel's blockade, according to the Iranian state broadcaster.