US Secretary of Navy Ray Mabus is in New Zealand and is expected to discuss whether the United States will send a ship to the Royal New Zealand Navy's 75th anniversary later this year.

In a statement, Mabus said the invitation for the United States to send a vessel to those celebrations was expected to come up in his discussions with New Zealand military leaders, but no decision had yet been made by the United States.

"Yes, the United States has received an invitation to the Royal New Zealand Navy anniversary celebrations planned for this coming November. No decision has been made yet and we do not have a timeframe on when a decision will be made."

Secretary Mabus said while it would be raised, it was not the purpose for his visit. It is Secretary Mabus's third visit to New Zealand since he was appointed to the role by President Barack Obama in 2009.

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He emphasised defence ties between New Zealand and the US were strong, and included partnering in humanitarian assistance, disaster relief and peacekeeping operations.

"In fact our relationship with New Zealand across the board continues to grow and we discuss and cooperate one wide range of issues at the highest levels."

It is the first formal invitation for a US naval vessel since the nuclear-free legislation passed in 1987. Since then no US vessels have visited although Mr Key said in 2012 he was open to a visit from a coastguard vessel. The US Marines also visited in 2012 to mark the 70th anniversary of the arrival of US World War II forces in New Zealand.

The US announced in 1991 and reaffirmed in 2004 that its vessels did not carry nuclear arms but has otherwise had a policy of refusing to confirm or deny the nuclear capabilities of its vessels.

Last week Mr Key said it was possible he could be assured of that without the US confirming it, depending on the type of vessel the US opted to send if it accepted the invitation.

Ship specifications can often be ascertained from open records.

Several countries have been invited to send ships, including China.