Mattis had harsh words for what he called “Russian influence operations” and efforts to pay off certain political groups.
“We don’t want to see Russia doing there what they have tried to do in so many other countries,” he said.
“No doubt that they have transferred money and that they are also conducting a broader influence campaign,” he said.
Russia is opposed to any Nato enlargement in eastern Europe and the Balkan, and Washington has accused Moscow of leading an online disinformation campaign in Macedonia to discourage voters from taking part.
Mattis said Washington and Skopje would expand “cybersecurity cooperation to thwart malicious cyber activity that threatens both our democracies”.
But prime minister Zaev played down his allegations of Russian meddling. “We don’t have proof of Russian influence,” he said earlier on Monday on the sidelines of a freedom of speech conference.
“The Russian Federation is a friend to Macedonia and they have nothing against our integration in the EU, but they are against our integration in Nato,” he said.
Montenegro, another tiny Balkan country, joined Nato in 2017, despite opposition by Moscow and part of the population.
But Laura Cooper, who is in charge of Pentagon policy on Russia and central Europe, accused Moscow of paying voters to boycott the referendum and of financially supporting pro-Russian organisations.
“They are swooping in now with disinformation and other forms of malign influence to try to change the minds of the Macedonian people,” she told reporters.
“There is this influence campaign to try to buy off people and try to support pro-Russian organisations.”