A concerned group of former foreign ministers and secretaries of state, including New Zealand's Donald McKinnon and America's Madeleine Albright, have penned a letter to US President Donald Trump warning him about his "deteriorating relationship" with allies.
The letter, absent of former Australian foreign ministers' signatures, was sent prior to Trump departing for Europe and the UK for expected rocky talks with leaders of NATO member nations.
Trump accuses NATO members of underfunding the security alliance and unfair trade with the US.
"We are gravely concerned about the deteriorating relationship between the United States and its Western allies," the 16 foreign ministers, who meet regularly, wrote in the letter sent to the White House.
"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."
Trump is expected to meet with Putin one-on-one and behind closed doors in Finland at the end of his week-long visit.
The foreign ministers said a successful NATO Summit would strengthen America's hand ahead of the Putin meeting.
Trump, however, kept up his tough line against NATO by firing off a tweet while flying to Europe on Air Force One.
"Many countries in NATO, which we are expected to defend, are not only short of their current commitment of 2% (which is low), but are also delinquent for many years in payments that have not been made," Trump wrote.
"Will they reimburse the U.S.?"