Prime Minister John Key had a surprise meeting with United States President Barack Obama at a lunch at the United Nations in New York today.

The pair were scheduled to meet for the first time at a US Presidential reception this evening but Mr Obama approached the Prime Minister unexpectedly at a lunch hosted by UN Secretary-General Ban Ki-Moon.

The pair chatted for two or three minutes.

Mr Obama asked Mr Key how he was enjoying the UN. The pair exchanged pleasantries before the US president moved on.

Mr Key will have another chance to meet the US leader at tonight's reception.

Security will be incredibly tight as most of 166 world leaders attending the UN General Assembly will attend.

New York Police Department is setting up a 10 block "freeze zone" around the Metropolitan Museum of Art where the function is being held.

The closest media can approach is four blocks.

This morning Mr Key told reporters he had bought a new purple tie for the big moment.

He only expected a quick greeting rather than what is known in diplomatic circles as a pull aside where more substantive issues could be discussed.

"I don't have high expectations. I mean there's 166 leaders here, the pressure on his time is unbelievable and realistically I've been of the view there will be more time at Apec and I think that's the way the Americans would see it as well."

Mr Key said he had been impressed by Mr Obama's speeches at the UN and Clinton Global Initiative over the past two days.

Mr Obama's speech to the United Nations general assembly was direct and of substance, he said.

But he was more reserved about Mr Obama's speech on climate change yesterday.

Mr Key initially said the president's presence was important but later said it was disappointing that there was not more substance.

Today, Mr Obama's first address to the general assembly since becoming president focused on the need for the world to work together to solve problems. He also spoke against nuclear weapons and urged peace between Israel and Palestine.

"I thought on balance he was a bit more direct today," Mr Key told reporters.

"I was (impressed), I thought he spoke really well and I thought he had a bit more substance today ... I think the speech was flawless.

"Yesterday I just don't think he actually came out with a clear and articulate programme of what the United States was going to do."