Prime Minister John Key says he was invited to the United States nuclear security summit in Washington because President Barack Obama recognised the importance of New Zealand's anti-nuclear position.

However, Mr Key is not expected to make any reference to New Zealand's anti-nuclear laws in a statement he presents to the summit taking place in Washington.

That is understood to concentrate more on treaties and conventions and Security Council resolutions that New Zealand has signed up to, or is planning to, in efforts to combat terrorism and prevent the proliferation of weapons of mass destruction.

The summit began early this morning, New Zealand time, and ends tomorrow morning.

Mr Key told the Herald that Mr Obama had raised New Zealand's anti-nuclear credentials with him in their conversation in May last year, a month after Mr Obama's Prague speech setting his goal of ridding the world of nuclear weapons.

"The first time President Obama rang me, the first thing he raised was New Zealand's anti-nuclear stance and the important role that that played in the world."

He had said it was something that he really cared about and that he would like to visit some day.

In correspondence that followed Mr Key said New Zealand stood ready to do whatever it could to help make the world a safer place.

And, when they met at the United Nations in September, Mr Obama told him he was organising the summit and would invite New Zealand.

"It is recognition that it is unique to New Zealand and it has marked us out," said Mr Key.

Most of the countries attending the summit have nuclear weapons or a nuclear energy programme, are involved in production of nuclear material, or are susceptible to nuclear smuggling across their borders.

It is expected to come up with some form of commitment to place all nuclear materials under secure storage within four years.

Chile, for example, recently sent all its highly enriched uranium - which was used at civilian reactors but can be used for nuclear weapons - to be stored in the United States.

Speaking to reporters yesterday, Mr Obama said the central focus of the summit "is the fact that the single biggest threat to US security - both short-term, medium-term and long-term - would be the possibility of a terrorist organisation obtaining a nuclear weapon".

"We know that organisations like al Qaeda are in the process of trying to secure a nuclear weapon - a weapon of mass destruction that they have no compunction at using.

"This is something that could change the security landscape of this country and around the world for years to come."

New Zealand's anti-nuclear policy, banning nuclear powered or armed warships, goes back to 1984 and it was enshrined in law in 1987, beginning 20 years of diplomatic strain with the United States.

The thaw began under the last Labour Government and continues.

Mr Key said New Zealand's relationship with the United States was "going from strength to strength".

He was scheduled to meet Vice President Joe Biden early today.

Mr Biden is a friend of the new US ambassador to New Zealand, David Huebner, and nominated him for the post.

Later today Mr Key will host a dinner at the New Zealand Embassy and among those on the guest list is Assistant Secretary of State for East Asia and the Pacific Kurt Campbell.

At the dinner, Mr Key will present the long-serving president of the US-NZ Council, John Mullen, with an honorary ONZM (Officer of the New Zealand Order of Merit).