Sources said Singapore would need to contact the Chinese Ministry of Foreign Affairs to get the vehicles back, adding that the discovery had been reported to Beijing already and the decision on whether to release them was no longer in Hong Kong hands.

According to a report by the FactWire news agency on Saturday, the cargo ship carrying the cars had set sail from Taiwan, but docked in Xiamen en route to Hong Kong, where the military kit was found.

Quoting customs sources, FactWire said that, before the ship arrived in the city, mainland law enforcers informed their Hong Kong counterparts that the vessel contained undeclared military materials, and did not have an approval notice. When it arrived in Hong Kong, customs officers searched the vessel and confiscated military material.

Singapore’s Ministry of Defence said in a statement on Saturday the vehicles were shipped in secure containers which were opened by the Hong Kong customs for inspection, and they had since been resealed.

On Friday, a spokesman for the Hong Kong Customs and Excise Department said the 12 Singapore-bound containers that carried “suspected controlled items” were found on board the vessel at Kwai Chung container terminal during a “routine ship search” on November 23.

The stand-off – in which Hong Kong is playing man in the middle – comes as China adopts an increasingly hostile attitude towards Singapore, which has recently had strategic geopolitical disagreements with Beijing over the likes of the South China Sea.

Experts have said that the vehicles, which had been in Taiwan while Singapore held military drills there, could be seen as additional blowback for four decades of military co-operation between Taiwan and the city state, which an extremely unhappy Beijing has tolerated, while officially seeing Taiwan as a renegade province.

The impounding of the nine advanced combat vehicles has triggered frantic diplomatic manoeuvres between Hong Kong, Beijing, Taipei and Singapore.

An armed forces team representing Singapore’s Ministry of Defence was due to arrive in Hong Kong on Friday night to “address the security of the equipment”, which has since been impounded at a customs facility in Tuen Mun.

Xiamen customs authorities could not be reached for comment on why they didn’t seize the vehicles themselves, tipping off their Hong Kong counterparts instead.

An officer from the General Administration of Customs in Beijing said Chinese customs had the right to detain military logistics carried on foreign ships that docked at mainland ports if they had not been declared in advance.

Lee Chih-hong, a research fellow at the Longus Institute for Development and Strategy in Singapore, said China might have allowed the seizure to happen in Hong Kong to embarrass Singapore whilst lessening the damage to the two countries’ ties.

“There is no doubt that the Chinese government wants to embarrass Singapore, but Beijing also doesn’t want to change the overall situation of Sino-Singapore diplomatic relations, so they let Hong Kong customs expose and handle this issue,” Lee said.

“China just wants to put pressure on Singapore over its stance in the South China Sea.”

A Hong Kong customs spokesman said it would not comment on enforcement operations.

Singapore’s defence ministry has been contacted for comment.

Additional reporting by Sidney Leng