Washington: Former US secretary of state John Kerry has accused President Donald Trump of "strange" and "counterproductive" behaviour at the outset of a NATO gathering and said Trump is "steadily destroying our reputation in the world."

The unusually harsh assessment from the country's former top diplomat came after Trump said at a breakfast meeting with NATO Secretary General Jens Stoltenberg in Brussels that he considered Germany to be "captive to Russia" for buying Russian gas, which he said was “inappropriate”.

“It certainly doesn’t seem to make sense that they pay billions of dollars to Russia and now we have to defend them against Russia,” he said.

Trump also continued his attacks on other US allies for their levels of defence spending.

Stoltenberg said it had been appropriate and understandable for defence spending to fall after the end of the Cold War, but the alliance had now recognised the need to bring it back up, with member countries adding billions to their defence budgets.