SKOPJE, Macedonia — NATO Secretary-General Jens Stoltenberg told Macedonia's political leaders Thursday that the Balkan country can 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 change of Macedonia's name to North Macedonia.

The proposal came after years of discussions with Greece, which has fretted over the name Macedonia ever since Yugoslavia broke up in the early 1990s.

Greece has argued that the name Macedonia implies a territorial claim against the Greek region of Macedonia and its ancient heritage. As a member of NATO, it has for years vetoed attempts by Macedonia to join the alliance.

Despite agreement at the highest levels of government, conservative opposition parties in both countries remain firmly opposed to the name-change 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 "absolute and total delusion."

Zaev said he is 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 Thursday, 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.

Later in the day, the NATO chief flew to Athens for a meeting with Greek Prime Minister Alexis Tsipras.

French Foreign Minister Jean-Yves Le Drian was also in Athens Thursday, 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.