Foreign ministry spokesman Geng Shuang said China made "representation" to Singapore after nine combat vehicles were stopped by Hong Kong customs en route from Taiwan on Wednesday. Geng called on Singapore to abide by Hong Kong laws and the "one China" principle.
"The Chinese government resolutely opposes nations with diplomatic ties with China to have any form of official contacts with Taiwan, including military exchanges and cooperation," he said. "We demand Singapore strictly abide by the one-China principle."
Observers said the protest was a warning to both Singapore and Taipei, which have seen their relationships with Beijing deteriorate.
Beijing accuses Singapore of being intractable over the South China Sea disputes.
It also cut off official communication with Taiwan after Tsai Ing-wen from the independence-leaning Democratic Progressive Party came into power in May.
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APL, the company hired by the Singapore Armed Forces to transport the military vehicles, said in a statement it was cooperating with Hong Kong authorities and was working with stakeholders.
Military ties between Singapore and Taiwan go back more than four decades. In 1974, Singapore and Taipei initiated "Project Starlight", which gave Singaporean troops much-needed physical room to carry out exercises on the self-ruled island.