A Palestinian girl who lost five family members in what medics said was Israeli shelling during an Israeli ground offensive, weeps outside a hospital in Beit Lahita in the northern Gaza Strip July 30, 2014. [Photo/Agencies]

GAZA/JERUSALEM  - Israeli shelling killed at least 15 Palestinians sheltering in a UN-run school and another 17 near a street market on Wednesday, Gaza's Health Ministry said, with no ceasefire in sight after more than three weeks of fighting.

Israel attacks symbols of Hamas power in Gaza

Israel's security cabinet decided to continue its offensive in the enclave and there was no sign of a halt to a 23-day conflict in which 1,346 Palestinians, mostly civilians, have died. On the Israeli side, 56 soldiers and three civilians have been killed.

Some 3,300 Palestinians, including many women and children, were taking refuge in the school in Jabalya refugee camp when it came under fire around dawn, the United Nations Relief and Works Agency (UNRWA) said.

"Our initial assessment is that it was Israeli artillery that hit our school," UNRWA chief Pierre Krahenbuhl said in a statement after agency representatives visited the scene and examined fragments, craters and other damage.

Blood splattered floors and mattresses inside classrooms at the Jabalya Girls Elementary School and survivors picked through shattered glass and debris for flesh and body parts to bury.

"I call on the international community to take deliberate international political action to put an immediate end to the continuing carnage," Krahenbuhl said.

The Gaza Health Ministry put the number of dead in the school attack at 15, with more than 100 wounded. The United Nations said 16 people were killed.

An Israeli military spokeswoman said militants had fired mortar bombs from the vicinity of the school and troops shot back in response. The incident was still being reviewed.

The army said three Israeli soldiers were killed on Wednesday when a booby-trap bomb exploded in a tunnel shaft they had uncovered in a residence in the southern Gaza Strip.

UNRWA said on Tuesday it had found a cache of rockets concealed at another Gaza school - the third such discovery since the conflict began. It condemned unnamed militant groups for putting civilians at risk.

Krahenbuhl said the Jabalya school's precise location and the fact that it was sheltering thousands of displaced people had been communicated to the Israeli military 17 times, with the last notification just hours before the fatal shelling.

UN Secretary-General Ban Ki-moon, speaking in Costa Rica, condemned the killing. "It is outrageous. It is unjustifiable. And it demands accountability and justice," he said.

At the White House, National Security Council spokeswoman Bernadette Meehan said: "We are extremely concerned that thousands of internally displaced Palestinians who have been called on by the Israeli military to evacuate their homes are not safe in UN-designated shelters in Gaza.

"We also condemn those responsible for hiding weapons in United Nations facilities in Gaza."