UNITED NATIONS -- More than 123,000 people have been internally displaced in Gaza as a result of ongoing Israeli attacks, said the UN Office for the Coordination of Humanitarian Affairs (OCHA) on Monday.

Israeli airstrikes and shelling have hit houses and apartment buildings, with four large residential towers in Gaza City destroyed. Six healthcare workers have been killed and four others injured, with seven healthcare facilities and nine ambulances damaged, said the office.

Damage to water, sanitation and hygiene facilities has undermined services to more than 400,000 people in Gaza. The Gaza Power Plant is now the only source of electricity and could run out of fuel within days, it warned.

The UN Agency for Palestine Refugees (UNRWA) estimates that more than half of those displaced are sheltering in dozens of schools. UNRWA has designated emergency shelters to host them and provide critical aid, said the office.

Cash assistance is also urgently needed for both displaced people and host communities in Gaza. Humanitarian partners are working to provide dignity kits and psychosocial support for affected families, it added.

The World Food Programme (WFP) has begun distributing food for up to 100,000 internally displaced people in Gaza, who are seeking refuge in UNRWA shelters, with fresh bread and canned food, said Stephane Dujarric, spokesman for UN Secretary-General Antonio Guterres.

In the next few days, the WFP plans on starting to roll out assistance to up to 800,000 people with food and cash assistance as the situation develops, provided the necessary funding is made available. The WFP needs 16.8 million U.S. dollars to reach 805,000 people in the next month, he told a daily press briefing.

The aid was from pre-positioned supplies, which are not endless. At some point in the not-too-distant future, the world body will run out of supplies unless more can come in, warned the spokesman.

Guterres on Monday said he was deeply distressed by Israel's announcement that it would initiate a complete siege of the Gaza Strip, with nothing allowed in - no electricity, food, or fuel.

The humanitarian situation in Gaza was extremely dire even before the sudden escalation of violence over the weekend. Now it would only deteriorate exponentially, he warned at a press conference.

Medical equipment, food, fuel and other humanitarian supplies are desperately needed, along with access for humanitarian personnel. Relief and entry of essential supplies into Gaza must be facilitated, he said. "I urge all sides and the relevant parties to allow United Nations access to deliver urgent humanitarian assistance to Palestinian civilians trapped and helpless in the Gaza Strip. I appeal to the international community to mobilize immediate humanitarian support for this effort."

An UNRWA school sheltering displaced families was directly hit over the weekend. The school was severely damaged and was housing about 225 people, though no casualties were recorded among the displaced, said Dujarric, the spokesman.

All UNRWA schools across the Gaza Strip are closed. More than 300,000 students are impacted, he said.

Dujarric said UNRWA has 13,000 staff members in Gaza, the vast majority of them are national staff.

The UNRWA staff are not able to leave Gaza. They are focused on trying to do whatever they can to help the population within their mandate, said the spokesman.