The 25-year stand-off with the United States over New Zealand's 'no nukes' policy is all but over.

American Secretary of State Hillary Clinton will make a "significant announcement" about a new, warmer relationship between the two countries, on her visit to New Zealand next month.

The Ministry of Foreign Affairs and the US State Department are working on the wording of the announcement, which will be made jointly with Prime Minister John Key. It is expected to include new levels of security cooperation.

The Herald on Sunday encountered retired senior diplomat Brian Lynch - now director of Victoria University's New Zealand Institute of International Affairs - arriving at Washington DC's Dulles International Airport on Sunday, and he is likely to contribute to discussions with US officials and analysts.

Ernest Bower, a director of the Centre for Strategic and International Studies, said in DC today that he had been briefed on the planned announcement.

He expected it would be a significant step towards ending the chill, though it was unlikely to formalise any new military alliance. Anzus, the three-way alliance between the US, Australia and New Zealand, is not expected to be renewed.

Instead, New Zealand and the United States are likely to lift the ban on joint military exercises and operations - a ban that the two countries have already sidestepped on a number of occasions.

Mr Bowers said he would like to see Barack Obama speed up moves to free trade as well.

"I'm frustrated that a guy with the communication skills of Barack Obama has not taken the opportunity to sell an issue like trade to the Americans," he said.

"If you don't engage with the world your children are going to be at a competitive disadvantage in the future."

The thaw in the chilled relationship began three years ago when George W Bush was president, but Clinton's visit will formalise the new relationship.

There is no suggestion that New Zealand will agree to visits from nuclear-powered or nuclear-armed warships or submarines. Indeed, the USA removed nuclear weapons from all its surface vessels some time ago.

Mr Bower said the two countries needed to gradually build up confidence in a renewed relationship, allowing their defence forces to work together on tasks as diverse as Pacific regional maritime security, humanitarian assistance and disaster relief.

"We want to move the relationship to a new level," he said. "This will be a significant announcement."

He said the relationship with Australia was already "rock solid", based on common values and common interests. New Zealand and the US needed to work towards the same goal. "But what's missing is there's a trust gap from the end of the alliance."