Updated

NATO Secretary-General Jens Stoltenberg told Macedonia's political leaders Thursday that the Balkan country could only join the military alliance if voters back a name change that will placate concerns within neighboring Greece.

Stoltenberg met with Prime Minister Zoran Zaev to express support for his "yes" campaign in the Sept. 30 referendum over the proposed name change of Macedonia to North Macedonia.

The proposal came after years of discussions with Greece, which has fretted over the name Macedonia ever since the former Yugoslavia broke up in the early 1990s. Greece has argued that the name Macedonia implied a territorial claim against the Greek region of Macedonia and ancient heritage.

As a member of NATO, Greece has for years vetoed attempts by Macedonia to join the alliance. Despite the agreement at the highest levels of government, conservative opposition parties in both countries remain firmly opposed to the agreement. Protests against the deal are expected at the weekend in the northern Greek city of Thessaloniki.

"This is a once in a lifetime opportunity," Stoltenberg told reporters in Skopje. "There is no way you can join NATO without the name agreement."

To believe otherwise, he said, was an "absolute and total delusion."

Zaev said he was confident of victory in the referendum, citing recent opinion polls.

Western leaders have strongly backed Zaev's campaign. Austrian Chancellor Sebastian Kurz and German Chancellor Angela Merkel are due to visit Skopje Friday and Saturday.

In central Skopje, Stoltenberg attended a ceremony to rename a street after his father, the late Norwegian politician Thorvald Stoltenberg.

As a young diplomat, he had helped coordinate a major international relief effort in the wake of a devastating earthquake in Skopje in 1963.

The NATO chief is due later in the day in Athens for a meeting with Greek Prime Minister Alexis Tsipras before leaving Friday.

The French Foreign Minister Jean-Yves Le Drian was also in Athens to discuss plans by Macedonia and other Western Balkan nations to deepen integration with the rest of Europe and NATO, as well as other regional security issues. The minister met with his Greek counterpart, Nikos Kotzias.

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Gatopoulos reported from Athens.