A group of 16 former foreign ministers, including New Zealand's Sir Don McKinnon, have written to US President Donald Trump concerned about the deteriorating relationship between the United States and its Western allies and Trump's overtures to Russia.

The letter comes ahead of a Nato summit in Brussels and Trump's meeting with Russian President Vladimir Putin, and urges Trump to reverse the "dangerous trend".

The letter, dated July 9, was signed by McKinnon - a former New Zealand foreign minister, deputy prime minister and later Commonwealth secretary-general and 15 other former foreign ministers.

They include Madeleine Albright of the US, Joschka Fischer of Germany, and David Milliband and Malcolm Rifkind of the UK.

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"We are gravely concerned about the deteriorating relationship between the United States and its Western allies. We are writing to you now, ahead of the Nato Summit and your meeting with Russian President Vladimir Putin, to underscore the need to reverse this dangerous trend," they wrote.

McKinnon told Newstalk ZB today that the group of foreign ministers, which met recently in France, felt it was necessary to tell Trump that institutions such as Nato should not be torn down.

"He's made a number of comments about the WTO [World Trade Organisation], about Nato, concerned that they are just not serving the interests of the United States. Well, in fact they do serve the interests of the United States, they do serve the interests of the world and certainly world stability. It would be a very difficult world if the WTOs and the Natos are no longer considered relevant," McKinnon said.

"You just need to listen to some of the comments that he's made about the fact that of all the Nato allies, none of them are pulling their weight enough, He's clearly holding out his hand to Mr Putin which is an interesting gesture."

The 16 former foreign ministers are a group which meets regularly to discuss threats to international security and prosperity.

"Over time, the gap between American and European defence investment has put the long-term health of the [Nato] alliance at risk. You have rightly expressed concern, as your predecessors did, about this disparity. But now, alliance members have responded," they said in the letter.

Nato is a grouping of European and North American countries established following the end of WWII. The letter says it played an "essential role in containing the Soviet military threat during the Cold War.

The letter said Russia was trying to divide the Nato alliance and its actions in Crimea and Eastern Ukraine violated international law. Its interference in elections around the world had undermined confidence in democracy.

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It urged Trump to show his support for Nato at the upcoming summit.

"To ignore or accept these actions would be a grave mistake, inciting further aggression and instability."