Israeli forces have encircled the Hamas stronghold of Gaza City, Israel's military says, as it continues its assault on the Gaza Strip.

The Israeli Defence Forces (IDF) said soldiers were engaged in close quarters combat with Hamas fighters staging hit-and-run attacks from tunnels.

The UN said four of its schools being used as shelters had been damaged and warned water shortages were worsening.

The Hamas-run health ministry says over 9,000 Palestinians have been killed.

On Thursday the UN Relief and Works Agency (UNRWA) said four of its schools being used as shelters hade been damaged in less than 24 hours.

At least 20 people were reportedly killed at a school in the Jabalia refugee camp, UNRWA said, while a child was reportedly killed at a school-turned-shelter in the Beach refugee camp.

The BBC has verified two videos from the schools.

One extremely graphic video, filmed at an elementary school in the Jabalia refugee camp in northern Gaza, shows the aftermath, first outside the main gate of the school and then in the main courtyard.

At least 20 people, either dead or injured, can be seen on the ground, including men, women and children.

The second video, filmed at the main courtyard of a primary school in Beach refugee camp, also in northern Gaza but near the Mediterranean coast, shows a plume of smoke rising from behind the main building of the school.

Several people, including children, run for safety in the courtyard, while some residents of the camp watch from the windows above. A series of explosions can be heard in the background of the video. The last one, the loudest, causes the crowd in the courtyard to flee in panic.

Hamas authorities in Gaza blamed Israeli air strikes. The IDF has not yet commented.

UN-appointed experts have called for a humanitarian ceasefire in Gaza, saying time was running out for Palestinian people there who find themselves at "grave risk of genocide".

"The situation in Gaza has reached a catastrophic tipping point," they said, warning that a ban on fuel entering Gaza and disruption to water supplies meant people had little access to safe drinking water.

"Water is essential to human life and today, two million Gazans are struggling to find drinking water," they said.

The Israeli mission to the UN in Geneva called the comments "deplorable and deeply concerning" and blamed Hamas for civilian deaths.

Earlier on Thursday the IDF said it had killed about 130 Hamas fighters.

Israeli PM Benjamin Netanyahu said troops were at "the height of battle".

"We've had impressive successes and have passed the outskirts of Gaza City. We are advancing," he said in a statement.

A number of foreign nationals were able to leave Gaza after the Rafah crossing with Egypt opened for a second day. President Biden says more than 70 Americans were among them.

But medical aid organisation Doctors Without Borders says more than 20,000 wounded people remained trapped in the territory.

Meanwhile, the Lebanese movement Hezbollah said it had attacked 19 targets in Israel simultaneously, in what would be its most intense assault on Israel so far.

The Israeli military said it was striking a series of Hezbollah targets in Lebanon in response.

It comes a day before a much-anticipated speech by the leader of Hezbollah Hassan Nasrallah, his first public address since the conflict broke out last month.