With anger in his voice, UN chief Ban Ki-moon criticised world leaders who keep "feeding the war machine" in Syria in his last speech before the General Assembly as secretary-general.

Ban, whose term ends on December 31, said at the opening of the annual meeting on Tuesday that "powerful patrons" on both sides in the more than five-year Syrian conflict "have blood on their hands".

"Present in this hall today are representatives of governments that have ignored, facilitated, funded, participated in or even planned and carried out atrocities inflicted by all sides of the Syria conflict against Syrian civilians," he said.

"Many groups have killed innocent civilians - none more so than the government of Syria, which continues to barrel bomb neighbourhoods and systematically torture thousands of detainees."

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The secretary-general also angrily denounced those responsible for an attack on an aid convoy delivering relief supplies to Syrian civilians near Aleppo on Monday. At least 20 people were killed in the strike. 

"Just when we think it cannot get any worse, the bar of depravity sinks lower," Ban said. "The humanitarians delivering life-saving aid were heroes. Those who bombed them were cowards."

Ban also accused South Sudan's feuding leaders of betraying their people.

He expressed regret for sexual abuse by UN peacekeepers in Central African Republic and an outbreak of cholera in Haiti.

"The despicable acts of sexual exploitation and abuse committed by a number of UN peacekeepers and other personnel have compounded the suffering of people already caught up in armed conflict, and undermined the work done by so many others around the world," Ban said.

"Protectors must never become predators," he said.

The Paris climate deal on curbing greenhouse gas emissions stands out as Ban's crowning achievement [Justin Lane/EPA]

In Haiti, Ban is working on a new response to an outbreak of cholera. The country was free of cholera until 2010, when UN peacekeepers dumped infected sewage into a river.

On a more personal note, Ban said he was stepping down as the world's number one diplomat with the conviction that the United Nations can help improve people's lives.

"After 10 years in office, I am more convinced than ever that we have the power to end war, poverty and persecution," he said.

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The Paris climate deal on curbing greenhouse gas emissions stands out as Ban's crowning achievement, an ambitious accord that he defended early on in the global push to address climate change.

"We have no time to lose," Ban told the gathering. "I urge you to bring the Paris Agreementinto force this year."

Source: News Agencies