According to officials, an Israeli airstrike on Thursday hit three houses in Khan Younis in southern Gaza Strip, killing 10 Palestinians, including an infant, and injuring 15, Xinhua reported.
Also Thursday, four Israeli tanks shelled a school run by the UN Relief and Works Agency for Palestine Refugees (UNRWA) in the northern Gaza Strip town of Beit Hanoun which killed 16 civilians and injured dozens who had taken refuge there.
Witnesses said that the four shells directly hit the school where dozens of civilians were sitting in classrooms and children playing in the yard.
Beit Hanoun's mayor said that the targeted school had hosted some 1,200 displaced local residents who had fled from Israeli bombardment.
A Hamas spokesman in Gaza later said that targeting a UNRWA school was "a big war crime".
Israel will pay a very heavy price for committing all these crimes against women and children.
An official with the Palestine Liberation Organisation (PLO) also said that attacking the school was "a big shame for humanity and for this fascist Israeli government".
But the Israeli military Thursday night said that the strike on the school was in retaliation to the shooting at them from the vicinity of the school.
Initial inquiries into the incident showed that militants opened fire from the school area at Israeli soldiers who later responded with heavy fire, the Israeli military added.
The military Spokesperson's Unit said the army had warned civilians to evacuate from the area and even authorized a humanitarian time window for evacuation, but "Hamas prevented the civilians from leaving it and once again used their infrastructure and international symbols as human shields".
UN Secretary-General Ban Ki-moon said he was "appalled" by the attack which "underscores the imperative for the killing to stop".
Israeli tanks shelled a school run by the UN Relief
He said among the dead were "women and children, as well as UN staff".
The health ministry in Gaza Thursday said that the ongoing Israeli offensive on the strip, which started in July 8, has already killed at least 800 Palestinians, two thirds being civilians including women and children, and injured more than 5,000 others.
Earlier Wednesday the UN human rights agency decided to form a committee to look into possible Israeli war crimes in Gaza.
Israel Thursday arrested some 150 alleged Hamas members in Gaza's Rafah area near the border with Egypt.
Though Israel has also lost four civilians and 28 troops since its military operation in Gaza started over two weeks ago, its Defence Minister Moshe Ya'alon said he instructed the troops Wednesday to prepare for a wider ground offensive.
Despite the mass civilian casualties in Gaza, the ongoing international efforts to broker a ceasefire between Israel and Hamas-led militants still seemed to have not yielded any substantial progress.
US Secretary of State John Kerry arrived in Jordan Thursday to discuss the latest developments in Gaza.
Both sides of the meeting voiced their support to Egypt in brokering a ceasefire and underlined the need for continued coordination in the progress.
Kerry was earlier in Egypt and Israel to push for a truce in Gaza.