Beijing on Monday warned Singapore to be careful about its “remarks and actions” as the island nation said it had written to Hong Kong’s leader asking for the return of a batch of armoured personnel carriers “taken hostage” en route from Taiwan.

Responding to media inquiries, Chief Executive Leung Chun-ying’s office said Hong Kong customs was the authority tasked with investigating the matter.

It also confirmed receipt of the letter from Singapore Prime Minister Lee Hsien Loong.

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Beijing, already irked by Singapore’s position on the South China Sea territorial disputes, called on the city state to be cautious in its handling of the incident.

Chinese foreign ministry spokesman Lu Kang said Monday that the Hong Kong government was handling the incident according to legal procedures, after Singapore Defence Minister Ng Eng Hen said the detention of the vehicles did not comply with international law.

“I hope the relevant parties can be cautious in their remarks and actions,” Lu said.

“I want to stress that China hopes other nations, including Singapore, follow the one-China principle. This is the foundation for bilateral ties between China and any other nation.”

Earlier yesterday, Ng said that the Singapore Armed Forces Terrexes and other equipment detained in Hong Kong were the property of the Singapore government.

“They are protected by sovereign immunity, even though they were being shipped by commercial carriers,” Ng told lawmakers in parliament.

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He added: “This means that they are immune from any measures of constraint abroad. They cannot legally be detained or confiscated by other countries. This principle is well established under international law ... the Singapore government has asserted our sovereign rights over the Terrexes.

“The Hong Kong authorities have responded that the investigation is ongoing and will take some time to complete, and that the Hong Kong government will handle the matter in accordance with their laws. Singapore welcomes this response.”

The nine Terrex vehicles and other equipment on board a commercial container ship were seized by Hong Kong customs on November 23.

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Hong Kong authorities have so far not issued formal reasons for the seizure, although initial reports suggested the shipping firm did not prepare the relevant documentation needed.

The episode triggered a sharp reaction from Beijing, which has routinely voiced its displeasure over Singapore’s close military ties with Taiwan.

Additional reporting by Eddie Lee