BEIJING (AP) — China on Friday threatened to retaliate against Lithuania after the Baltic nation allowed Taiwan to open a representative office in its capital, Vilnius.

Foreign Ministry spokesperson Zhao Lijian said Lithuania would “reap what it sows,” but gave no details.

Zhao described Lithuania's move as an “egregious act” that “grossly interferes” in China’s internal affairs.

The office, which opened Thursday, bears the name Taiwan rather than “Chinese Taipei,” which is used by the International Olympic Committee and many foreign nations to avoid offending China, which claims the self-governing island democracy as its own territory.

Taiwan has just 15 formal diplomatic allies, but maintains informal ties with all major nations through trade offices that act as de facto embassies, including in the United States and Japan.

It wasn't clear what actions China plans to take in response to the opening of the office. Beijing has already recalled its ambassador from Vilnius and expelled the Lithuanian ambassador.

Lithuania plans to open a representative office in Taipei by the end of the year and has withdrawn from the “17 plus one” arrangement launched by China to bind it closer to countries in Eastern Europe.

China's threat underscores its extreme sensitivity to any challenge to what it considers its “core interests," as it presses ahead with its increasingly assertive foreign policy.