China's Foreign Ministry on Monday said they hope Singapore can respect the "One China Policy" after Singapore's Prime Minister Lee Hsien Loong sent a letter to Leung Chun-ying, Hong Kong's Chief Executive, demanding the nine Singaporean armored cars the city detained six weeks ago to be returned immediately.

The military equipment was seized en route back home after finishing a routine training in Taiwan on November 23, 2016.

Carried by the commercial shipping company of APL, the nine cars were scheduled to arrive in Singapore on November 29.

Lu Kang, spokesman for China's Foreign Ministry, said the "One China Policy" is the basic prerequisite for the country to develop relations with any other countries.

He also said Singapore is expected to comply with the law in Hong Kong.

The Hong Kong government said their investigation is underway and it will take some time.

Singapore's Defense Minister Ng Eng Hen said the military equipment belongs to the Singaporean government and thus enjoys the sovereign immunity under international law. He said Hong Kong customs' action went against both its indigenous law and the international law, and the city is due to return the armored cars.

Singapore's Foreign Minister Vivian Balakrishnan said they wouldn't let the accident kidnap bilateral relations, yet he argued that their military arrangement with Taiwan is not a secret, but has long been known to all. Ng Eng Hen further said they would reclaim the cars and continue overseas training after the investigation completes.

However, according to Hong Kong's Import and Export (Strategic Commodities) Regulations, armored cars and tanks are categorized as strategic commodities that can only enter or leave the city with a permission signed by the Trade and Industry Department. The Singaporean armored cars hadn't provided such documents when passing through Hong Kong.

Ng said the cars, worth about 21 million US dollars, involve no ammunition or sensitive materials and can be openly purchased on the market.