Israeli tank shells hit a compound housing a UN school in the northern Gaza Strip on Thursday, killing at least 15 people and wounding dozens who were seeking shelter from fierce clashes on the streets outside.

Pools of blood soiled the school courtyard, amid scattered books and belongings. There was a large scorch mark in the courtyard marking the place where one of the tank shells hit.

The strike occurred during a day of heavy fighting throughout the coastal territory as Israel pressed ahead with its operation to halt rocket fire from Gaza and destroy a sophisticated network of cross-border tunnels.

The Gaza health official Ashraf al-Kidra said the dead and injured in the school compound were among hundreds of people seeking shelter from heavy fighting in the area.

It was the fourth time a UN facility has been hit in fighting between Israel and Palestinian militants in Gaza, since the Israeli operation began on 8 July.

UNRWA, the Palestinian refugee agency, has said it has found militant rockets inside two vacant schools.

The strike came on a day of heavy fighting throughout the Gaza Strip as Hamas militants stuck to their demand for the lifting of an Israeli and Egyptian blockade amid international efforts to broker a ceasefire.

A Palestinian boy cries after being evacuated from the compound hit by Israeli shells. Photograph: Mohamed Salem/Reuters

Six members of the same family and an 18-month-old boy were killed when an Israeli air strike hit the Jebaliya refugee camp in the early morning hours, according to Gaza police and health officials. Twenty others were injured in the strike, they said, and rescuers were digging through the rubble of flattened homes, looking for survivors.

An air strike on a home in the southern Gaza town of Abassan killed five members of another family, said Kidra, the Gaza health official. Abassan is near Khan Younis, in an area that saw intense fighting on Wednesday.

The 16-day conflict has claimed the lives of more than 700 Palestinians, most of them civilians, Palestinian health officials say. Israel has lost 32 soldiers, all since 17 July, when it widened its air campaign into a full-scale ground operation.

Two Israeli civilians and a Thai worker in Israel have also been killed.

Israel's prime minister, Binyamin Netanyahu, made no reference to the ceasefire efforts in underscoring his determination to neutralise the rocket and tunnel threats.

More than 2,000 rockets have been fired at Israel from Gaza since 8 July, and the Israeli military says it has uncovered more than 30 tunnels leading from Gaza to Israel, some of which have been used by Hamas to carry out attacks.

"We started this operation to return peace and quiet to Israel … And we shall return it," Netanyahu said at a joint appearance with the visiting British foreign secretary, Philip Hammond.