U.S. Defense Secretary Jim Mattis arrived in Macedonia on September 17 with a warning that Russia was attempting to use its money and influence to build opposition to the upcoming referendum on changing the former Yugoslav republic’s name.

"No doubt that they have transferred money and they are also conducting broader influence campaigns," Mattis told reporters traveling with him to the Macedonian capital, Skopje.

He said it was unclear how effective Moscow's efforts to defeat the referendum had been.

Western leaders have visited Skopje in recent days to back a "yes" vote at the September 30 referendum on whether to change the Balkan country's name to North Macedonia.

The planned vote follows an agreement between Skopje and Athens in June to break a decades-long stalemate that has poisoned their relations since 1991 and could pave the way for Macedonia to join NATO -- a move Moscow opposes.

Opinion polls have shown the “yes” vote with a clear lead, but it will require a turnout of more than 50 percent of the Balkan nation’s 1.8 million registered voters to be valid.

In Skopje, Mattis met Macedonian Defense Minister Radmila Sekerinska and center-left Prime Minister Zoran Zaev, who struck the name-change deal with Athens.

Mattis said that agreement with Greece unlocked Macedonia's NATO accession process and allowed Macedonians to determine their own future in "institutions made up of like-minded countries."



"You would join an alliance in which countries large and small work together to uphold shared principles of national sovereignty, territorial integrity, and freedom from coercion, while others seek to diminish these very values, sowing discord from Syria to the Ukraine," Mattis said.



Mattis called the referendum a "complex political choice" and "the most important vote" in the nation's history. ​

WATCH: U.S. Defense Secretary Jim Mattis urged Macedonians to vote in a forthcoming referendum on changing their country's name, after arriving in Skopje on September 17.

The U.S. defense secretary later announced a plan for the United States and Macedonia to "expand our cybersecurity cooperation to thwart malicious cyberactivity that threatens both our democracies."

Zaev predicted that Macedonians will vote in favor of the name change, saying, "There is no other alternative for the Republic of Macedonia than the integration into NATO and the EU."



Mattis also held talks with President Gjorge Ivanov, who opposes the agreement with Greece.

Moscow's ambassador to Skopje has criticized Macedonia's ambitions to join NATO, warning that the Balkan country could become "a legitimate target" if relations between NATO and Russia deteriorate further.

In July, Greece expelled two Russian diplomats and blocked two others from entering over suspicions they attempted to undermine the deal between Skopje and Athens.

Moscow denounced the expulsions as unjustified and responded in kind with expulsions of Greeks.

The name dispute between Macedonia and Greece dates back to 1991, when Macedonia peacefully broke away from Yugoslavia, declaring independence as the Republic of Macedonia.

Greece has objected to the name Macedonia, saying it implies territorial claims on the northern Greek region with the same name.

Greece, an EU and NATO member, has cited the dispute to veto Macedonia's bid to join the two organizations.

With reporting by Reuters, AFP, and AP