New Zealand's long deadlock over American naval ship visits could soon end, with the first formal invitation extended to the United States since anti-nuclear legislation was introduced in 1987.
The invitation is to attend the Royal New Zealand Navy's 75th anniversary commemorations in November next year.
Prime Minister John Key said the government had invited a range of countries to visit - including China and the US - and it was up to those governments to decide whether they would attend.
"If in the event that any country decides to send a ship, and the standard practice at the moment, whether it's a military celebration like this or just because they're in New Zealand, I'm required to sign a piece of paper which means that I believe that they have met the conditions of the New Zealand law."
The US government announced in 1991 it would withdraw all tactical nuclear weapons from its surface ships and attack submarines.
That was reiterated by the US in 2004, "that all nuclear weapons have been removed from ground force and naval surface vessels - in the sum the army, marine corps and surface and air component of the navy have been denuclearised".
Mr Key said New Zealand was not changing its law.
"So the only condition would be that they come to New Zealand - and I'm in the position to sign that certificate of verification - is that they're neither nuclear-powered.
"It's a standard process, it happens all the time, it's just a bit more high profile because of a potential US ship."
Mr Key said he personally would like to see a US ship come to New Zealand.
"In so much that if you think about the, you know, kind of, stand-off over this issue, I mean at the time it was very significant and the relationship took quite a knock for a long period of time.
"But if you really think about the state of the relationship right now, it's probably never been in better shape, so this is really just the last vestiges, if you like, of that particular disagreement.
He said the government had no intention of changing the anti-nuclear law, "but I don't see why that should inhibit what is otherwise a great relationship".
The US had long adopted a policy of neither confirming nor denying its nuclear capabilities on its vessels, but the general view was that there was now enough open source information available about which ships were nuclear-propelled that New Zealand could find out without a direct request to the US.
The US Embassy said the invitation has been received, but a decision had not yet been made.
A spokesperson said the relationship with New Zealand was strong across the board and continues to grow, "and we discuss and cooperate on a wide range of issues at the highest levels".
The Ministry of Foreign Affairs and Trade said the possibility of a visit had been "touched on" in discussions the Foreign Minister has had with the US over recent years, but as far as it was aware this was the first formal invitation.
The invitation itself was seen as a significant breakthrough, and was unlikely to have been offered if it was going to result in a stand-off between New Zealand and the US.
The Ministry of Foreign Affairs and Trade said a decision on how the US would be represented at the commemorations was not expected until closer to the time.
No declaration necessary, says PM
If the US accepted the invitation to send a naval ship here, New Zealand would not necessarily have to demand a direct assurance it was not nuclear-powered, Mr Key said.
The US has had a long-standing policy of neither confirming nor denying its nuclear activity.
But there were other ways of confirming a vessel's status, Mr Key said.
"There's plenty of open source documentation and qualification that would allow you to form a view - for instance, I don't think anyone's ever argued that a US coastguard is either nuclear-powered nor nuclear-armed.
"There's enough stuff there, depending on the vessel that they send, for an assessment to be made."
The driving force behind the invitation was not to secure a US ship visit to New Zealand, but rather to celebrate the navy commemoration, he said.
"And in doing that, it would seem very odd to me that we wouldn't invite the Americans when we would invite many, many other navies from around the world.
"Whether the US decide to participate, or if they don't, for whatever reasons, ultimately is something that's got to be worked through.
"But, you know, if you take a step back it just strikes me - no-one needs a rehearsal of the issues - but it strikes me, we've worked our way through all those.
"The relationship's in amazing shape."
In the end, it was up the Americans to decide whether or not to come, he said.
"If they do and if they come, it's great and I think New Zealanders would, you know not all of them, but many of them would celebrate - it's a matter for the Americans ultimately."