Mr Tutu had an emotional meeting with members of the Assamna family
South African Nobel peace laureate Desmond Tutu is on a UN fact-finding mission to Gaza, focusing on the deaths of Palestinians caused by Israeli fire.
He met relatives of 19 civilians killed in the Israeli shelling of two houses in Beit Hanoun in 2006.
Mr Tutu's team will report its findings to the UN Human Rights Council in Geneva, a body shunned by Israel.
In the latest violence, Hamas said two of its men were killed in an Israeli air strike in southern Gaza.
The army said it had conducted two air strikes targeting militants firing mortar shells.
The former archbishop of Cape Town called Israel's blockade of the Hamas-controlled territory illegal, and urged Palestinian militants to halt cross-border rocket attacks.
Israel says the Beit Hanoun deaths in November 2006 were a mistake during action to target areas used by Palestinian militants.
Mr Tutu listened to surviving members of the Assamna family, from which most of the 19 victims, who included eight children, came from.
"I was here with my son. I was holding his hand when he died," Tahini al-Assamna told Mr Tutu.
"Can you imagine a mother holding the intestines of her own son?" she said through her tears.
Mr Tutu said the purpose of the visit was to gather information, "but we wanted to say that we are quite devastated".
Israel views the UN Human Rights council as biased against it and is not co-operating with Mr Tutu's mission.
The Israeli authorities denied him a visa, so he entered Gaza via Egyptian territory, rather than the usual route through the Erez checkpoint, which Israel controls.
Israel tightened its restrictions at Gaza's border crossings after Hamas took over the territory nearly a year ago.