On September 30, Macedonians vote on whether to change the country's name to the Republic of North Macedonia, which could open the door to NATO and EU membership.

The planned vote follows a landmark deal on the change signed by Athens and Skopje in June in a bid to break a stalemate that has poisoned their relations since 1991 and hobbled Macedonia's integration with the West.

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

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

"I think to the people whose lives can be changed by economic opportunities, by security among... 30 democratic nations, I think it's very important that they have those options available," Mattis said on the plane taking him to Skopje.

But "this is a decision for our Macedonian friends. And however they go, you know that we are going to live by it."

Mattis met his Macedonian counterpart Radmila Sekerinska and Prime Minister Zoran Zaev, who struck the name-change deal with Greece.

Speaking to reporters after the meeting, he announced a plan for Washington and Skopje to "expand our cybersecurity cooperation to thwart malicious cyber activity that threatens both our democracies".

Washington accuses Russia, which opposes NATO enlargement in eastern Europe and the Balkans, of leading a disinformation campaign in Macedonia through social media to discourage voters from taking part.

- 'Russian influence operations' -

The Pentagon chief slammed what he called "Russian influence operations" in the country of 2.1 million.

"We don't want to see Russia doing there what they have tried to do in so many other countries," he said.

Mattis also accused Russia of sending money to influence some political groups.

"No doubt that they have transferred money and that they are also conducting a broader influence campaign."

Montenegro, another tiny Balkan country, joined NATO in 2017, despite the opposition of Moscow and a part of the population.

Laura Cooper, in charge of Russia and central Europe at the Pentagon, accused Moscow of paying voters to boycott the referendum and 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," Cooper told reporters.

"There is this influence campaign to try to buy off people and try to support pro-Russian organisations."

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.

In an interview with Macedonian news portal Nova Makedonija in late August, Russian ambassador to Skopje 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 campaign for a 'Yes' vote, which seems the likely outcome.

According to the Macedonian constitution, the referendum will only be "consultative" -- meaning that a 'Yes' outcome will need to be backed by parliament with a two-thirds majority.

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

"We'll just look at how they'll shape their own future, not shaped by someone else," said Mattis, the first US defence chief to visit Skopje since 2004.