Defense Secretary Jim Mattis arrived in Macedonia Monday, condemning Russian efforts to use its money and influence to build opposition to an upcoming vote that could pave the way for the country to join NATO - a move Moscow opposes.
Mattis told reporters travelling with him to Skopje that there is "no doubt" that Moscow has been funding pro-Russian groups in order to defeat the referendum on a name change later this month.
"They have transferred money and they're also conducting broader influence campaigns," Mattis said.
"We ought to leave the Macedonian people to make up their own minds."
Macedonians will vote Sept 30 on whether to approve the new name of North Macedonia is an effort to placate Greece, which has for years blocked Macedonia's path to NATO and the European Union.
But any progress toward NATO membership by the Balkan nation is strongly opposed by Russia, which doesn't want the alliance to expand to areas formerly under Moscow's influence.
Mattis is the latest in a string of international leaders visiting Macedonia to voice support for the referendum, and he's the most senior US official to go there.
NATO Secretary-General Jens Stoltenberg, German chancellor Angela Merkel, and Austrian chancellor Sebastian Kurz have all visited and made public endorsements of the name change, saying it's critical in order for the country to join NATO, after years of waiting.
Mattis said that he and other NATO allies "say right up front in open press what we think.
We're not passing money to people behind the scenes, we're not putting together parties that we control or try to control."
Russia has already been called out for trying to influence the vote.
In July, Greece expelled two Russian diplomats accused of supplying funds to protest groups who were opposing the name change deal. Russia denounced the expulsions as unjustified.
Greece, a member of NATO, has for years vetoed attempts by Macedonia to join NATO, complaining about the country's name ever since Yugoslavia broke up in the early 1990s. Greece argues that the name implies a territorial claim against the northern Greek region of Macedonia and its ancient heritage.
NATO leaders in July formally invited Macedonia to begin membership talks on the condition that it wouldn't become effective until the name change was implemented.
But there is widespread concern about Russian impact on the vote.
"There is this influence campaign to try to buy off people and try to support pro-Russian organizations," said Laura Cooper, the deputy assistant defense secretary for Russia and the region.
She said she couldn't give specifics about the pay-offs, but said the US is aware of financial support that Moscow has given to pro-Russian individuals and groups that are working to undermine the referendum.
Russia, she said, is "swooping in now with disinformation and other forms of malign influence to try to change the minds of the Macedonian people."
As examples, she cited efforts to convince people that the vote isn't relevant and isn't the last step for NATO admission.
Evelyn Farkas, an expert on the region who is a fellow with the Atlantic Council and a former Defense Department adviser, said Mattis' visit to the tiny nation could help sow support for the name change.