In his final address to world leaders at the UN General Assembly in New York, secretary-general Ban Ki-moon appealed to "all those with influence, to end the fighting" in Syria.
"Many groups have killed many innocents, but none more so than the government of Syria, which continues to barrel bomb neighborhoods and systematically torture thousands of detainees," he told the General Assembly.
"Powerful patrons that keep feeding the war machine also 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 Syrian conflict against Syrian civilians."
With just three months to go before he steps down as UN chief, Ban also made a final plea to end the "madness" of the Israeli-Palestinian conflict and gave world leaders advice on good governance.
"My message to all is clear: serve your people. Do not subvert democracy, do not pilfer your country's resources, do not imprison and torture your critics."
The former South Korean foreign minister is stepping down on December 31 after 10 years in what has been widely described as the world's most impossible job.
Now in its sixth year with over 300,000 dead, the war in Syria is dominating this year's gathering of world leaders and stands out as the unfinished business of Ban's tenure.
"I appeal to all those with influence to end the fighting and get talks started," Ban said.
Denouncing the "sickening, savage and apparently deliberate attack" on an aid convoy in Syria, Ban said: "Just when you think it cannot get any worse, the bar of depravity sinks lower."
The UN chief hailed the aid workers on the convoy to Aleppo province as "heroes" and said "those who bombed them were cowards" before calling for accountability for crimes committed in the war.
He blamed all sides for killing innocent people, but "none more so than the government of Syria, which continues to barrel bomb neighborhoods and systematically torture thousands of detainees."
Taking a swipe at "powerful patrons" fueling the conflict, Ban said these governments "in this hall today" had "ignored, facilitated, funded, participated in or even planned and carried out atrocities" in Syria.
No time to lose on climate
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 Agreement into force this year."
The push for early entry into force of the Paris deal got a shot in the arm this month when the United States and China, the world's top two polluters, ratified the deal.
Describing climate change as the "defining challenge of our time," Ban said only 26 countries representing 15 percent of emissions must join the agreement for it to enter into force.
Addressing the Israeli-Palestinian conflict, Ban said it pained him that the past decade had been "lost to peace" with Israel building more settlements and the Palestinians facing growing hopelessness.
"This is madness. Replacing a two-state solution with a one-state construct would spell doom," he warned.
After North Korea carried out its fifth nuclear test on the back of a series of missile launches, Ban urged Pyongyang to "change course" and fulfil its obligations to scrap its military programs.
Switching to French, the UN chief expressed regret over the "despicable" sexual abuse and rape committed by UN peacekeepers and for the cholera epidemic in Haiti that has killed thousands since it broke out in 2010 near a UN peacekeepers' base.
The United Nations will put forward a package of assistance for cholera victims and help Haiti build sound water, sanitation and healthy systems, he said.
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 peoples' lives.
"After ten years in office, I am more convinced that ever that we have the power to end war, poverty and persecution," he said.
