Singapore's defense minister says nine armored personnel carriers that have been detained in Hong Kong should be returned immediately under sovereign immunity laws.

Customs authorities in Hong Kong, a Chinese territory, seized the eight-wheeled Terrex infantry carriers in November. The vehicles were being shipped back to Singapore from Taiwan.

Defense minister Ng Eng Hen said Monday in parliament that the vehicles are protected by sovereign immunity.

The incident appears to signal China's unhappiness with Singapore's position on Taiwan — which China considers a breakaway region — and the South China Sea.

Although not a claimant in the South China Sea, Singapore has irritated China by advocating that countries should abide by international rules. Despite a landmark ruling against its claims, China maintains ownership over the entire strategic waterway.