US Defense Secretary Jim Mattis announced plans Tuesday to visit Macedonia, ahead of a referendum that could end a years-long spat with Greece over the country's name and pave the way for it to join Nato.

The Pentagon chief said he was concerned Russia might be attempting to interfere in the outcome of the election, which if successful could also eventually see Macedonia join the EU.

"Democracies should be left alone," Mattis told reporters at the Pentagon.

"If their ambassador wants to make a statement in the paper, that's one thing. But the kind of mischief that Russia has practiced from Estonia to the United States, from Ukraine and now to Macedonia, it always is adapted to the specific situation and it's always beyond the pale, as far as I'm concerned."

He added: "I'm going there to make very clear we stand with the Macedonian people."

Mattis's trip to Macedonia next weekend comes after NATO in July formally invited the Balkan nation to join the Western defense alliance.