Singapore on Monday demanded the immediate return of nine armoured troop carriers impounded by Hong Kong while in transit from Taiwan, calling the seizure illegal under international law.
The November seizure of the Terrex vehicles by Hong Kong customs has triggered a row also involving China, which has sovereignty over Hong Kong and considers Taiwan a renegade island awaiting reunification.
The vehicles were being shipped home from military exercises in Taiwan, where land-starved Singapore has for decades trained its troops.
Following the seizure China lodged a diplomatic protest with Singapore over its military cooperation with the island.
Defence Minister Ng Eng Hen said Singapore had repeatedly told the Hong Kong government that the vehicles and other equipment "belong to the government of Singapore and are therefore immune from any measures of constraint.
"Accordingly we have requested the Hong Kong authorities to return our property immediately," he told parliament.
The vehicles are protected by sovereign immunity even if shipped by commercial carriers, Ng said.
"They cannot legally be detained or confiscated by other countries."
Ng said Prime Minister Lee Hsien Loong had written to Hong Kong Chief Executive C.Y. Leung on the issue, and Hong Kong authorities had replied that the investigation would take time.
Hong Kong seized a total of 12 containers, saying they carried "suspected controlled items".
The Singapore military has reviewed its shipping procedures to ensure its equipment will not be "taken hostage en route" in the future, Ng added.
Singapore's demand for the vehicles' return triggered a quick rebuke from Beijing, with Chinese foreign ministry spokesman Lu Kang urging the island nation to "abide by the one-China policy".
"As far as I'm concerned, (Hong Kong) is dealing with this case in accordance with relevant laws and regulations, and we hope all parties can be prudent with their words and actions," Lu said.
Ties between China and Singapore are already strained over Singapore's perceived support for Southeast Asian nations disputing China's extensive territorial claims in the South China Sea.
A 1974 agreement allows Singaporean troops to train in Taiwan alongside Taipei's forces, and it sends up to 15,000 troops a year to the island.
Hong Kong is semi-autonomous after being handed back by Britain to China in 1997 and enjoys freedoms unseen on the mainland, but there are increasing fears over Beijing's interference.
China controls foreign affairs issues under the 1997 agreement.