Slated for September 30, the vote will ask Macedonians whether they want to change the country's name to the Republic of North Macedonia, in a move which could open the door to NATO and EU membership.

The referendum follows a landmark agreement between Athens and Skopje in June which sought to break the stalemate that has poisoned their relations since 1991 and hobbled Macedonia's integration with the West.

Greece objects to its neighbour being called Macedonia because it has its own province of the same name.

It has also accused Skopje of territorial ambitions and blocked the Balkan country from joining NATO or starting EU accession talks.

Speaking on the plane en route to Skopje, Mattis said it was up to the Macedonians to decide whether they wanted the change, which could open important doors for this country of 2.1 million.

"I think to the people whose lives can be changed by economic opportunities, by security... it's very important that they have those options available," he said.

"We'll just look at how they'll shape their own future, not... someone else."

- 'Russian influence operations' -

But he 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 Balkans, and Washington has accused Moscow of leading an online disinformation campaign in Macedonia to discourage voters from taking part.

On arrival, Mattis -- the first US defence chief to visit Skopje since 2004 -- met his Macedonian counterpart Radmila Sekerinska and Prime Minister Zoran Zaev, who struck the name-change deal with Greece.

Speaking to reporters after the talks, Mattis said Washington and Skopje would expand "cybersecurity cooperation to thwart malicious cyber activity that threatens both our democracies".

- 'No proof of Russian influence' -

But Zaev played down his allegations of Russian meddling.

"We don't have proof about Russian influence," he told reporters earlier on Monday on the sidelines of 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."

- Macedonia's 'consequential mistake' -

In June, Russian ambassador to the EU Vladimir Chizhov told Greek media that Macedonia's choice to become a NATO member was "a mistake".

"There are errors that have consequences," he said.

Speaking to Macedonian news portal Nova Makedonija in late August, Russian ambassador Oleg Shcherbak accused the West of putting "very strong media and psychological pressure" on Macedonian voters.

Macedonian media, notably the main TV channels, have led a 'yes' campaign, which seems the likely outcome.

Under the Macedonian constitution, the referendum is only "consultative" -- meaning that any vote in favour of the change will need to be backed by parliament with a two-thirds majority.

The nationalist opposition VMRO-DPMNE, which opposes the name change, has decided not to back a social media campaign calling for a boycott of the vote.