The violence of the Gaza conflict has spread to the West Bank where six Palestinians have reportedly been killed and scores wounded in the past 24 hours in the biggest clashes with Israeli forces for several years.

The Israeli military was put on high alert in the West Bank on Friday, which has been declared a "day of rage" by Palestinian factions.

Three Palestinians were shot dead in the occupied West Bank in separate incidents involving both the Israeli army and a civilian who appeared to be a Jewish settler, medics and witnesses said.

In the first incident, 46-year-old Hashem Abu Marieh was killed in the Palestinian village of Beit Ummar near the flashpoint southern city of Hebron by Israeli soldiers, medical sources said.

A 26-year-old man was also reported to have died in Hebron from gunshot wounds.

In the second incident, a group of settlers opened fire on protesting Palestinians after they threw stones at their car near the northern West Bank city of Nablus, Palestinian security sources said. An 18-year-old Palestinian named as Khaled Oudeh was killed.

Shortly afterwards, Israeli troops arrived at the scene and clashed with the Palestinians, firing live bullets and tear gas. The Israeli army fire killed a second Palestinian, 22-year-old Tayyib Oudeh, the security sources said, adding that three other Palestinians were injured by live fire.

Israeli soldiers uncover a Palestinian tunnel in Gaza Strip. Photograph: IDF spokesman/Rex

Friday's violence followed major clashes on Thursday, when 20,000 people took part in a march from Ramallah towards East Jerusalem in protest over the bloodshed in Gaza. Two Palestinians were killed and several hundred injured in clashes with Israeli soldiers, with 120 treated for gunshot wounds.

The UN secretary general, Ban Ki-Moon, has called for an immediate "humanitarian pause" as the fighting continued unabated in the Gaza Strip, where Israeli air force jets struck 30 homes in the Gaza Strip on Friday morning, killing a leader of the Palestinian militant group Islamic Jihad and his two sons. Israeli ground troops and Hamas fighters engaged in intense gun battles in the north and central stretches of the coastal enclave. The Israeli military confirmed the death of a 36 year-old reservist.

Israel reported that 35 rockets were fired from Gaza, 10 of which were intercepted by its Iron Dome missile defence system.

Ban's call came after several meetings with the US secretary of state, John Kerry, and Egyptian officials in Cairo aimed at ending the 18-day conflict that has killed more than 845 people, most of them Palestinian civilians.

"On this, the last Friday of Ramadan, I call for an immediate, unconditional humanitarian pause in the fighting in Gaza and Israel. This pause would last through the Eid al-Fitr holiday period," Ban said in a statement released by his office.

The Israeli security cabinet was due to review the latest ceasefire proposal from Kerry. It was also to discuss the option of expanding its eight-day-old ground operation in Gaza.

As the negotiations continued, French lawyer Gilles Devers announced he had lodged a complaint at the International Criminal Court on behalf of the Palestinian justice minister accusing the Israeli army of war crimes.

"Israel, the occupying power, is carrying out a military operation which in principle and form violates the basis of international law," he said.

"Every day new crimes are committed and over 80% of the victims are civilians. Children, women, hospitals, UN schools … the Israeli soldiers respect nothing.

"This is a military attack against the Palestinian population."

More than 15 women, children and United Nations staff were killed and about 200 injured on Thursday morning when Israel shelled a UN school sheltering hundreds of Palestinians fleeing the violence.

The Israel Defence Forces (IDF) insisted it had given the occupants of the shelter time to leave before shelling the area. But the UN flatly contradicted this, saying it had made repeated attempts to negotiate a window of time during which people could safely leave the area but none had been granted. It said it had given the IDF precise co-ordinates of the location of the school.

Reports emerged that Kerry had presented both sides with a new proposal for a cessation of violence. It centred around a week-long temporary ceasefire during which Israeli troops would be allowed to stay in Gaza to locate and destroy tunnels, and negotiations would be held for a permanent deal, with guarantees by the US, EU and UN that the primary concerns of each side would be addressed.

Kerry was said to be awaiting a response from Israel and Hamas before leaving Cairo to return to Washington later in the day. Israel's security cabinet was due to discuss the plan on Friday, while Hamas's leader-in-exile, Khaled Mishal, said a truce must include a guaranteed end to Israel's eight-year blockade of the Gaza Strip. "We want a ceasefire as soon as possible, that's parallel with the lifting of the siege of Gaza," he told the BBC.

The attack on the school in Beit Hanoun, in northern Gaza, was one of the grimmest incidents of the conflict, which is in its 18th day. More than 800 Palestinians, mostly civilians, have been killed, as well as 34 Israelis and one Thai worker.

The UN secretary general condemned the school attack, which came hours after the agency had warned that Israel's actions in Gaza could constitute war crimes.

"Today's attack underscores the imperative for the killing to stop, and to stop now," Ban said.

Valerie Amos, the UN's top humanitarian official, described the attack on the school as appalling.

The school was hit while its playground was crowded with families waiting to be ferried to safety. According to survivors, one shell landed in the schoolyard followed by several more rounds that hit the upper storeys of the building.

Most of the wounded were moved initially to a local hospital where a shell exploded about 50 metres away as terrified women and children waited for news of relatives.

More than 140,000 Palestinians have sought shelter in UN premises during the conflict. Several schools run by UNRWA, the UN agency for Palestinian refugees, have come under fire in the last week. On Tuesday, a school in Maghazi, central Gaza, which was sheltering about 1,000 people, was hit by Israeli shells as an UNRWA team inspected damage caused by an earlier strike.

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

The Israeli military, which said it was "reviewing the incident", claimed it had occurred during heavy combat in the area and accused "terrorists" of "using civilian infrastructure and international symbols as human shields".

In the West Bank clashes between the IDF and up to 10,000 people demonstrating against the Gaza offensive erupted in the volatile area around the massive Qalandiya checkpoint between Jerusalem and the West Bank city of Ramallah.

The IDF said it used "riot dispersal means" – a term used to cover weapons such as rubber bullets and teargas – against protesters, who threw rocks and molotov cocktails at them and blocked a road with burning tyres. It did not say whether live rounds had been used.

According to Israel Radio, the protest appeared to be the largest since the Palestinian second intifada, or uprising, which ended in 2005.

Two other Palestinians have been killed in confrontations in the West Bank this week. On Thursday night, police also clashed with Palestinian protesters around Jerusalem's Old City.