UN Secretary-General Antonio Guterres warned Wednesday against ongoing clashes in Israel and the occupied Palestinian territories spreading, and voiced alarm at reports of escalating attacks from Lebanon, according to Anadolu Agency.

"We must avoid spillover of the conflict," Guterres told reporters at the UN's New York City headquarters. "I am concerned about the recent exchange of fire along the blue line, and recent reported attacks from southern Lebanon."

"I appeal to all parties and those who have an influence over those parties to avoid any further escalation and spillover," he added.

Israeli forces launched a sustained and forceful military campaign against the Gaza Strip, a response to a military offensive by the Palestinian group Hamas in Israel.

The conflict began when Hamas initiated Operation Al-Aqsa Flood against Israel -- a multi-pronged surprise attack including a barrage of rocket launches and infiltrations into Israel via land, sea, and air.

Hamas said it was in retaliation for the storming of the Al-Aqsa Mosque in occupied East Jerusalem and Israeli settlers’ growing violence against Palestinians.

The Israeli military launched Operation Swords of Iron against Hamas targets within the Gaza Strip in response.

That has extended into cutting water and electricity supplies to Gaza, further worsening the living conditions in an area that has reeled under a crippling siege since 2007.

Guterres said: "Crucial life-saving supplies, including fuel, food and water must be allowed into Gaza.”

"We need rapid and any unimpeded humanitarian access now," he said.

The UN’s human rights chief, Volker Turk, said Tuesday that Israel’s “full siege” of the Gaza Strip violates international law.

“The imposition of sieges that endanger the lives of civilians by depriving them of goods essential for their survival is prohibited under international humanitarian law,” Turk said in a statement.

Guterres said UN personnel are on the ground in Gaza "working around the clock to support" civilians amid the spiraling violence, and about 220,000 Palestinians have taken shelter at 92 UNRWA facilities in the coastal enclave. UNRWA is the UN's Palestine refugee agency.

"UN premises, and all hospitals, schools and clinics must never be targeted," he said. "I deeply regret that some of my colleagues have already paid the ultimate price."

Nine UNRWA staffers have been killed in Israeli airstrikes in Gaza since Saturday, and 20 of the agency's facilities have been affected by airstrikes, including two schools.

Eleven UNRWA staffers have been killed in Israeli airstrikes in Gaza since Saturday, including some who died in their homes with their families, the agency said. Nearly two dozen of the agency's facilities have been affected by airstrikes, including two schools.