New York: United Nations Secretary-General Ban Ki-moon has accused the Syrian government of killing the most civilians during the country's five-year war and said "powerful patrons that keep feeding the war machine also have blood on their hands".

In his final address to an annual gathering of world leaders at the UN on Tuesday, Ban said the Syrian government "continues to barrel bomb neighbourhoods and systematically torture thousands of detainees".

Speaking after the Syrian government declared the US-Russian ceasefire over and resumed air strikes, Ban denounced the bombing of a humanitarian aid convoy headed for Aleppo in strong terms, his voice tense with anger.

"Just when we think it cannot get any worse, the bar of depravity sinks lower," Ban said in his last speech as leader of the organisation after a decade in the job, much of it preoccupied with Syria.