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The UN's top Mideast envoy warned on Thursday that the Gaza Strip is "about to explode" and declared the world "must do everything possible to prevent another war" between Israel and Palestinians.

His warnings came ahead of Friday's fifth weekly mass unarmed demonstrations in the Gaza Strip demanding the right of return for refugees and protesting Israel's devastating blockade.

The protests along the Gaza border quickly turned deadly after Israeli forces responded with live ammunition.

At least 41 Palestinians have been killed and thousands wounded by live ammunition and rubber-coated steel bullets.

Nickolay Mladenov called Gaza a "powder keg" and told Security Council members that "old wounds continue to bleed and deepen as we speak, risking the outbreak of another war."

Describing the current situation in Gaza, Mladenov used some of his strongest remarks yet.

"What is happening in Gaza is an injustice that no man, woman or child should have to endure," he said.

"People should not be destined to spend their lives surrounded by borders they are forbidden to cross, or waters they are forbidden to navigate. They should not be destined to live under the control of Hamas, which invests in militant activities at the expense of the population."

The UN envoy said he has engaged intensively with all sides to reduce "risks of friction" and welcomed efforts by Egypt and others.

"Israel must calibrate its use of force and minimize the use of live fire. Lethal force should be used only as a last resort," Mladenov said.

"Hamas and the leaders of the demonstrations must keep protesters away from the Gaza fence and to prevent all violent actions and provocations."

Riyad Mansour, the Palestinian UN ambassador, accused Israel's military forces of targeting civilians in Gaza and called their deliberate killing and wounding "terrorism."

He insisted the Friday demonstrations at the Gaza-Israel border were peaceful protests by Palestinians demanding "their dignity and freedom" — especially in Gaza where its two million inhabitants have been subjected to an Israeli land, sea and air blockade for a decade.

Israel's UN Ambassador Danny Danon accused Hamas of orchestrating provocations and confrontations along the security fence and using women and children as human shields.

Hamas "terrorists are hiding while allowing — even hoping — for their people to die," he said, calling this "evil in its purest form."

The United Nations has repeatedly warned that Gaza may become uninhabitable by 2020 due to Israel's blockade and three devastating military operations on the besieged territory since 2008.