The first visit to the United States by Royal New Zealand Navy vessels in 25 years is a further sign of a progressive thaw in relations but does not mean all is forgiven, says Defence Minister Wayne Mapp.

The frigate Te Kaha leaves today for a four-month trip which will include stopping in American ports in July.

The fleet tanker Endeavour is also on the deployment, which will be the Navy's first visit to the United States since 1985, when New Zealand's nuclear-free law put an end to such visits between the two countries.

The New Zealand ships will visit only civilian ports, in Seattle, San Francisco, San Diego and Honolulu, and there will be no military exercises with US forces.

Dr Mapp said planning for the trip began more than a year ago and preceded the US review of its relationship with NZ and the decision to relax the ban on joint exercises.

It was part of a progressive development in relations between the two countries since 2005, he said.

"It might have needed a waiver of some description but they were never going to be visiting a US naval facility. The relationship is not such that New Zealand vessels use United States naval facilities, just as US naval vessels don't visit New Zealand."

Dr Mapp said it was a fortuitous opportunity because the vessels were travelling to Canada first to celebrate the Canadian Navy's centenary.

The US at present has to grant a special waiver for New Zealand forces to take part in any joint exercises under the provisions of the ban that followed New Zealand's enactment of its anti-nuclear policy.

Royal NZ Navy spokesman Commander Phil Bradshaw said that while in North America, the vessels would host guests and hold open days as part of the Navy's role in promoting New Zealand.

Te Kaha and Endeavour will exercise with Singaporean, Malaysian and Australian forces in the South China Sea before travelling to Canada for the centenary.

This story incorrectly states that the visit of the RNZN frigate Te Kaha to the United States would be the first visit by a NZ Navy ship to the US in 25 years. The frigate Waikato took part in celebrations in Hawaii in 1995 to mark the end of World War II in the Pacific in 1945.