The International Criminal Court (ICC) has forced governments to alter their behaviour in the eight years of its existence, the UN chief has said.
Ban Ki-moon told a summit in Uganda discussing the Hague-based court that it had curtailed impunity and had broken new ground on victims' rights.
But he called on member countries to step up co-operation.
The ICC has five active investigations, all in Africa. So far no-one has been convicted of alleged war crimes.
Delegates from more than 100 countries are attending the meeting, to take stock of the ICC's achievements and push forward proposals for strengthening its rules.
"Few would have believed then that this court would spring so vigorously into life, fully operational, investigating and prosecuting war crimes and crimes against humanity," Mr Ban told the delegates, AFP news agency reports.
"In this new age of accountability, those who commit the worst of human crimes will be held responsible."
Mr Ban said the time had passed when the world faced a choice between peace and justice - now states had to pursue them hand-in-hand.
But the BBC's Karen Allen in the Ugandan capital, Kampala, says the issuing of arrest warrants against serving government leaders, in particular President Omar al-Bashir of Sudan for alleged war crimes in Darfur, has prompted some critics to argue that such indictments are a disincentive to achieving peace in the world's trouble spots.
After Mr Bashir's indictment last year the African Union said it would halt co-operation with the ICC.
However, the court's chief prosecutor Luis Moreno-Ocampo says he believes attitudes in Africa are changing.
"In Africa, what I feel is that there are some leaders who are trying to change, and some leaders who are opposing the change. And that's the tension," he told the BBC.
He called on the Western media to focus their attention on this change.
Mr Moreno-Ocampo said there had been a "legal revolution" since the Rome Statute - the international treaty that created the court.
He said it had affected the armed forces, governments and judges in many countries, citing the Democratic Republic of Congo, Guinea and Colombia.