WASHINGTON (Reuters) - U.S. Defense Secretary Jim Mattis said on Tuesday he would visit Macedonia before a Sept. 30 referendum on changing the country’s name, also expressing concern about suspected Russian interference in the vote, which Moscow denies.

FILE PHOTO: U.S. Secretary of Defence James Mattis addresses a press conference at the Ministry of Defence in Olso, Norway, July 14, 2018. Jim Watson/Pool via REUTERS

Macedonia scheduled the referendum on the government’s deal in June with neighboring Greece to change its name to the Republic of North Macedonia which would open the door for it to join NATO and the European Union.

“I am concerned about it... The kind of mischief that Russia has practiced from Estonia to the United States, from Ukraine and now to Macedonia, it always has adapted to the specific situation and it’s always beyond the pale,” Mattis, who will visit Macedonia over the weekend, told reporters.

Mattis said he wanted to make it clear the United States supported the Macedonian people.

NATO invited Macedonia to begin accession talks with the alliance, but said it would have to change its constitution and adopt the new name first. The EU has also said it would set a date for Macedonian accession talks pending implementation of the name deal.

Moscow’s ambassador to Skopje has criticized Macedonia’s ambitions to join NATO, saying it could become “a legitimate target” if relations between NATO and Russia deteriorate further.

Greece, a member of both NATO and the EU, has refused to accept the Balkan country’s name, saying it implies territorial claims on the Greek province of Macedonia and amounts to an appropriation of its ancient civilization.

Prime Minister Zoran Zaev’s government, elected in 2017, pushed for an agreement with Greece. Nationalists, including President Gjorge Ivanov, oppose the deal saying it is against the constitution.

In July, Greece expelled two Russian diplomats and barred two other people from entering the country, accusing them of having meddled by encouraging demonstrations and bribing unidentified officials to thwart the Macedonia agreement.

Russia has denied wrongdoing and responded in kind with expulsions of Greeks.