SINGAPORE: Singapore's defence minister on Monday said the nine armoured vehicles seized in Hong Kong could not be detained or confiscated, but that he welcomed Hong Kong's pledge for the dispute to be handled in line with its laws.

Hong Kong customs seized the troop carriers in November as they were being shipped to Singapore from Taiwan after military exercises on the island that Beijing regards as a breakaway province, sparking tension between Singapore and China.

Beijing, which regained sovereignty over the former British colony of Hong Kong in 1997, then warned countries against maintaining military ties with Taiwan.

The vehicles were Singapore property, said Singapore Defence Minister Ng Eng Hen, answering questions in parliament.

"They are protected by sovereign immunity even though they were being shipped by commercial carriers. They are immune from any measures of constraint abroad. They cannot legally be detained or confiscated by other countries," he said.

Singapore Prime Minister Lee Hsien Loong had written to Hong Kong chief executive Leung Chun-ying to request their return, Ng added.

The response was that an investigation would take some time and that Hong Kong would handle the matter in accordance with its laws, Ng said.

"Singapore welcomes this response," he said.

Hong Kong authorities confirmed the letter and their response, and said investigations were continuing.

At a regular briefing, Chinese Foreign Ministry spokesman Lu Kang told reporters that Beijing hoped "all relevant countries, including Singapore, first, can conscientiously abide by the one China principle.

"Second, we hope the Singapore side can conscientiously abide by Hong Kong's laws," he added.

Singapore could take legal action in Hong Kong courts based on the principle of sovereign immunity of military assets, said

a senior lawyer specialising in marine claims.

"I don’t see why it should be a big exercise to get the vehicles back," said the lawyer based in Hong Kong, who asked not to be identified, because the subject is a sensitive one.

The seizure came amid mounting regional uncertainty and signs of tension between China and Singapore, which has deepened its security relationship with the United States over the last year and remains concerned over Beijing's assertive territorial stance in the South China Sea.

Singapore and Taiwan have a military relationship dating to the 1970s, involving the use of Taiwan for Singapore infantry training.

Beijing has grudgingly tolerated this agreement since re-establishing diplomatic ties in the 1990s with Singapore, which recognises Beijing's "one China" policy that says Taiwan is part of its territory.

But China has repeatedly warned Singapore to stay out of the South China Sea dispute, where its claims overlap with those of Brunei, Malaysia, the Philippines, Taiwan and Vietnam.

Singapore has no claims in the South China Sea, but the open economy of Southeast Asia's biggest port depends on free navigation in the area.

Foreign Minister Vivian Balakrishnan, also speaking in parliament, reiterated Singapore's support for China.

"We must welcome a rising China, a stronger China, an economically more integrated China and we have to focus on the opportunities, while at the same time recognising that there will be issues to resolve from time to time," he said.

China has claimed sovereignty over Taiwan since 1949, when Mao Zedong's Communist forces won the Chinese civil war and Chiang Kai-shek's Nationalists fled to the island. Beijing has vowed to bring Taiwan under its rule, by force if necessary.