Israeli shells have struck a UN school in the Gaza Strip, killing at least 19 people and wounding scores more, after Israeli ground troops made a significant push into the territory.

Wednesday’s shelling of the Jabaliya refugee camp was the second time in a week that a UN school sheltering hundreds of homeless Palestinians had been hit.

Christopher Gunness, the UNRWA's spokesman, said the attack was a "source of universal shame" and blamed Israeli forces.

"We have visited the site and gathered evidence. We have analysed fragments, examined craters and other damage. Our initial assessment is that it was Israeli artillery that hit our school, in which 3,300 people had sought refuge.

"I condemn in the strongest possible terms this serious violation of international law by Israeli forces. I call on the international community to take deliberate international political action to put an immediate end to the continuing carnage."

Gunness said that UN representatives have informed Israeli forces about the exact location of the school 17 times.

Many of those in the school had fled their homes in northern Gaza after Israel dropped leaflets warning them of an "upcoming phase" of action.

The Israeli military said fighters near the UN school had fired mortar bombs and Israeli forces had shot back.

"Earlier this morning, militants fired mortar shells at [Israeli] soldiers from the vicinity of the UNRWA school in Jabaliya. In response, soldiers fired towards the origins of fire, and we're still reviewing the incident," a spokeswoman said.

Israel announced a four-hour "humanitarian window" on Wednesday, starting at midday GMT, but said it did not include areas where its soldiers were operating - about half of Gaza.

Hamas said it would not abide by the pause, unless its conditions were met - namely, the end of the blockade of Gaza.

At least 1,363 Palestinians have died in Israel's invasion, according to Gaza health ministry figures.

An official for UNRWA, the UN's Palestinian refugee agency, told the AFP news agency that the shelling hit a bathroom and two classrooms inside the girls' school.

Al Jazeera's Imtiaz Tyab, reporting from Gaza's Kamal Adwan hospital, where many of the injured were brought, put the number of wounded at more than 90.

"Looking around me I can see some with what appears to be shrapnel wounds and some with far more serious wounds," he said.

He said people there did not know why Israel had hit the shelter, adding that the attack caused panic among people living in different UN-run shelters.

"As we were driving to the hospital, we saw families with many children leaving other UN schools. They feel insecure. There seem to be no safe shelter for them, not even in UN schools," our correspondent said.

Nearly 220,000 displaced Palestinians have sought shelter in about 86 UNRWA schools, according to the agency.

The Israeli army had begun heavy tank shelling in the area a couple of hours prior to the incident.