SKOPJE, FYROM — NATO Secretary-General Jens Stoltenberg told leader of the Former Yugoslav Republic of Macedonia (FYROM) the country won’t get into the defense alliance unless is complies with a deal with Greece to change its name to North Macedonia.

Stoltenberg met Prime Minister Zoran Zaev to join in urging voters to back a referendum voter over the deal which ironically doesn’t mention the proposed new name, only if they want to ratify the agreement so the then-North Macedonia can join NATO.

Anti-nationalist Prime Minister and Radical Left SYRIZA leader Alexis Tsipras made the deal to end a 27-year name feud with FYROM that began when a New Democracy government allowed the new country emerging from the collapse of Yugoslavia to temporarily use the name Macedonia, that of an abutting ancient Greek province.

But after successive FYROM governments began claiming Greek lands, including the real Macedonia and second-largest city and major port of Thessaloniki, Greece used a veto as a member of NATO to bar FYROM’s entry and also European Union accession talks.

Tsipras agreed to lift both those barriers even though Zaev hasn’t moved yet to change his country’s Constitution as planned to remove irredentist claims on Greek land. The deal also allows FYROM citizens to identify as Macedonian, not North Macedonian, and have a Macedonian language and culture.

Despite agreement at the highest levels of government, conservative opposition parties in both countries remain firmly opposed to the name-change agreement – as does Tsipras’ junior coalition partner, the pro-austerity, marginal, jingoistic Independent Greeks (ANEL) of Defense Minister Panos Kammenos who said he will take his party out of the government if the deal is approved in FYROM and comes to a vote in Parliament.

“This is a once in a lifetime opportunity,” Stoltenberg told reporters in Skopje, FYROM’s Capital. “There is no way you can join NATO without the name agreement.” To believe otherwise, he said, was “absolute and total delusion.”

Zaev said he is confident of victory in the referendum, citing recent opinion polls. Western leaders, including US President Donald Trump, have strongly backed Zaev’s campaign.

Trump described the name deal between Greece and FYROM as an “historic” one that will boost security, stability and prosperity in the entire region. He said the agreement “resolves the long-standing name issue with Greece and paves the way for Macedonia’s membership in both NATO and the European Union.”

The United States calls FYROM by the name Macedonia, as do 140 other countries, one of the reasons Tsipras said he made the deal so that it would be called North Macedonia.

Austrian Chancellor Sebastian Kurz and German Chancellor Angela Merkel are due to visit Skopje Sept. 7-8.

In central Skopje, Stoltenberg attended a ceremony to rename a street after his father, the late Norwegian politician Thorvald Stoltenberg, who, as a young diplomat, he had helped coordinate a major international relief effort in the wake of a devastating earthquake in Skopje in 1963.

Stoltenberg later went to Athens to meet Tsipras. French Foreign Minister Jean-Yves Le Drian was also in Athens to talk about FYROM and other Western Balkan nations getting closer to NATO and the EU. The deal, if ratified, would let North Macedonia also open EU accession talks.

Tsipras, who, with 62 percent of Greeks opposed to the deal, has barred a referendum and if approved in FYROM the agreement would likely go to the Greek Parliament early in 2019 and precipitate a crisis that could bring down the government unless Kammenos backs off, as he usually does in dealing with Tsipras.

The deal was brokered with the help of UN envoy Matthew Nimetz, an American lawyer who had failed for two decades to find a solution before resuming talks earlier this year after a three-year break amid speculation the US pressured Greece to relent to get FYROM into NATO as an American bulwark against Russian interests in the Balkans.

(Material from the Associated Press was used in this report)