UPDATED 6.29pm: Prime Minister John Key has used his speech to the United Nations General Assembly to campaign for Helen Clark to become next UN Secretary General, describing her as a natural leader.

LISTEN ABOVE: UN correspondent Margaret Besheer spoke to Larry Williams about John Key, Syria and Obama at the UN

Mr Key has given a wide-ranging speech covering the Syria crisis, the Middle East peace process, North Korean missile tests, the Security Council veto and free trade.

But on the selection of the next Secretary General, he said it was an issue of critical importance to the future of the United Nations.

He said this is not the time for a business as usual appointment for Secretary-General.

Miss Clark, the former Labour prime minister, is still in the race despite poor results in polls taken by Security Council members.

Further polls will be taken before the council reaches a consensus on a candidate it can recommend to the General Assembly, and its five permanent members have the power of veto.

Miss Clark has said she intends being the last one standing after vetoes have been used to eliminate other candidates.

Mr Key said he had worked across the political divide with her for years.

"She rallies people together to find the common ground, even when the issues are difficult and the differences vast," he said.

"She gets things done. We think it's time for a secretary-general like Helen Clark."

Key also said the Security Council is not living up to its responsibilities.

The Prime Minister said the United Nations can't afford to have a weak council - it's simply not an option.

Mr Key said he's been deeply troubled to see the council, the pre-eminent body for international peace and security, failing to live up to its responsibilities on what he called the most serious crisis of our time, Syria.

Additional reporting from NZ Newswire