Jenin, West Bank - Israel killed three Palestinians in an air strike in the occupied West Bank on Friday, taking the death toll from a large-scale military operation now in its third day to at least 19.

A top UN aid official, meanwhile, questioned "what has become of our basic humanity," as Israel's carnage in Gaza rages on and humanitarian operations struggle to respond.

The United Nations has warned the military operation which Israel launched in the West Bank early on Wednesday is "fueling an already explosive situation" in the illegally occupied territory and has pressed Israel to end it.

In the US, Vice President Kamala Harris pledged she will not change Washington's policy of supplying weapons to Israel if elected to the top job in November, but she stressed it was time to "end this war."

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Israel has framed its raids on towns and refugee camps across the northern West Bank as "counter-terrorism" operations.

They have killed at least 19 Palestinians since Wednesday, the military and the Palestinian health ministry said.

The military claimed the three people it killed in an air strike near the northern city of Jenin on Friday were Hamas militants.

Witnesses told AFP the strike hit a car in the town of Zababdeh, southeast of the city.

Israeli troops pulled back from other West Bank towns late Thursday, but the attacks continued around Jenin.

The AFP journalist reported loud explosions from the city's refugee camp and thick plumes of smoke rising from the area.

In Gaza, Israeli artillery pounded western areas of Gaza City early Friday, an AFP journalist said, while a medical source at the southern Nasser Hospital said an Israeli strike killed three people near the southern city of Khan Younis.

The World Health Organization said Israel had agreed to at least three days of "humanitarian pauses" in parts of Gaza, starting Sunday, to facilitate a vaccination drive after the first case of polio in a quarter of a century was recorded in the territory.

Israeli Prime Minister Benjamin Netanyahu said the measures were "not a ceasefire" in the nearly 11-month, US-backed siege.

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In the West Bank, the Israeli army claimed it killed seven militants on Thursday, including five militants in Tulkarem refugee camp.

A military statement said one of the five was Muhammad Jaber, also known as Abu Shujaa, who Palestinian militant group Islamic Jihad said was its commander in the nearby Nur Shams refugee camp.

Two other militants were killed in Jenin on Thursday, the military said.

The Israeli assault has caused significant destruction, especially in Tulkarem, whose governor Mustafa Taqatqa described the raids as "unprecedented" and a "dangerous signal."

The Palestinian Prisoners' Club advocacy group said at least 45 people had been detained in the West Bank since Wednesday. An Israeli military spokesperson said "10 wanted individuals were arrested."

The United Nations said on Wednesday that at least 637 Palestinians had been killed in West Bank by Israeli troops or settlers since October 7. Nineteen Israelis, including soldiers, have been killed over the same period, according to Israeli official figures.

Last month, the International Court of Justice ruled Israel's decades-long occupation of the Palestinian Territories, including East Jerusalem, to be illegal.

Israeli shelling in the Jabalia refugee camp killed two people on Friday, Gaza's civil defense agency said.

The UN had to halt the movement of aid and aid workers within Gaza on Monday due to a new Israeli displacement order for the Deir el-Balah area, which had become a hub for its workers.

"More than 88% of Gaza's territory has come under an (Israeli) order to evacuate at some point," the acting head of the UN humanitarian office, Joyce Msuya.

She said civilians were being forced into just 11% of the Gaza Strip, already one of the most densely populated territories in the world before the war.

"What we have witnessed over the past 11 months... calls into question the world's commitment to the international legal order that was designed to prevent these tragedies," Msuya said.

"It forces us to ask: what has become of our basic sense of humanity?"

Israel has killed at least 40,602 people in Gaza since October, according to the territory's health ministry. The UN rights office says most of the dead are women and children.

The assault has devastated Gaza, repeatedly displaced most of its 2.4 million people, and triggered a humanitarian crisis and mounting accusations of genocide.

In central Gaza, some Palestinians returned to parts of Deir el-Balah after the military had amended a previous evacuation order.

Mohamed Abu Thuria told AFP he had "found massive destruction everywhere."

Another displaced Gazan back in Deir el-Balah, Ibrahim al-Tabaan, said: "We lost everything."