UN relief chief Tom Fletcher has denounced the growing violence against humanitarian workers, warning that the killing of aid staff in conflict zones such as Yemen and Palestine’s Gaza is becoming "normalised."

"We're under attack. Last year, more than 380 aid workers were killed, the highest ever. Two hundred and seventy already this year. It's a record that I fear will break again," Fletcher told reporters at UN headquarters in New York.

"Violence against us — in Yemen, and of course in the occupied Palestinian territories and Gaza — is somehow being normalised. It's an age of impunity."

He asked: "Where are these weapons coming from that are killing us and those we serve? They don't come out of nowhere."

Pressed on whether he was suggesting action against countries supplying arms to Israel, Fletcher replied: "Now I don't know about arms embargoes, and those other questions are there for the political colleagues. I don't want my people, our people, to be killed. That goes for the courageous humanitarian workers and those we serve."

Fletcher said aid workers are increasingly being killed "in sophisticated ways," including by drones and other advanced weapons.

He criticised Israeli rhetoric and attacks on civilian infrastructure in Gaza.

"We've heard Israeli ministers talking openly about flattening Gaza and forcing people out permanently, bombing food aid and so on," he said.

"I don't think that we have to choose between condemning the starvation of civilians in Gaza and demanding the unconditional release of hostages. We must do both," he added.

Asked about Hamas’ role, Fletcher said the UN has not faced major obstacles from the group.

"Our assessment is that Hamas are broadly staying out of the way of the aid distribution, which we do through community networks. We haven't had major obstacles from them."

Looting of convoys, however, remains a challenge. Fletcher said most incidents are driven by desperate civilians or armed gangs.

"If we could flood Gaza with hundreds of trucks a day, the looting would go away, the prices in the markets would come down," he said.

He also highlighted a sharp funding shortfall.

"Only 19 percent of the required funds have been received this year. That's a 40 percent drop from last year. Devastating. Makes it an age of indifference," Fletcher said.

"Twenty-nine billion dollars could save 114 million lives — just 1 percent of annual global defence spending."