China has voiced firm opposition to Vietnam's expanded island-building activities in the South China Sea, following a United States think tank report claiming Hanoi's construction in the Nansha Islands could rival or even surpass Beijing's projects.

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Responding to a question at a daily news briefing on Monday, Foreign Ministry spokesman Guo Jiakun reaffirmed that the Nansha Islands are China's inherent territory, adding that Beijing resolutely opposes construction activities on illegally occupied reefs and islands.

Guo warned that China will take "necessary measures to safeguard its territorial sovereignty and maritime rights".