Flags flew at half-mast at UN facilities across the globe on Monday including at the body's New York headquarters, as staff stood in silent tribute to the more than 100 colleagues killed in Gaza during the Israel-Hamas war.

The blue and white United Nations flag was lowered at 9:30 am local time at offices in Bangkok, Tokyo and Beijing, with other UN venues following suit.

Secretary-General Antonio Guterres led UN personnel in observing a minute of silence at UN headquarters when the clock struck 9:30 am in New York.

"Since the start of this conflict, more than 100 UNRWA staff have lost their lives – the highest number of UN aid workers killed in a conflict in such a short time," he posted on X, formerly Twitter, along with a photograph of senior UN officials sombrely standing in silence.

"They will never be forgotten."

The UN agency for supporting Palestinian refugees (UNRWA) announced on Friday that 101 of its employees had died in the Gaza Strip since the war erupted just over a month ago.

And on Sunday, a day before the worldwide tribute to fallen staff, the United Nations reported "a significant number of deaths and injuries" in strikes on one of its facilities in Gaza.

"UNRWA staff in Gaza appreciate the UN lowering the flag around the world," the agency director in the Gaza Strip, Tom White, said in a statement.

"In Gaza however, we have to keep the UN flag flying high as a sign that we are still standing and serving the people of Gaza."

Israel has been relentlessly bombarding the Gaza Strip since Hamas fighters carried out an October 7 attack on southern Israeli communities, the deadliest in the country's history.

About 1,200 people, mostly civilians, were killed in the Hamas attacks and around 240 people taken hostage, according to Israeli officials.

More than 11,000 people, most of them civilians and many of them children, have been killed in Gaza in retaliatory strikes by Israel, according to the territory's Hamas-run health ministry.

In the lobby of UN headquarters, the names of the staffers killed in Gaza were read out as dozens of colleagues listened. Some carried white sheets with the words "Stop the fighting" and "Protect civilians." (AFP)