BEIJING - China on Friday responded to Vietnam construction on a South China Sea island, urging the country to stop its occupation and illegal activities on China's territory.

"China is firmly opposed to the relevant country's illegal occupation of South China Sea reefs, and construction on China's territory," said Foreign Ministry spokesperson Geng Shuang at a routine press briefing.

Geng was responding to a question regarding a US think tank report, which said satellite images taken this month showed Vietnam had lengthened its runway on the Nansha Islands.

China has indisputable sovereignty over the Nansha Islands and their adjacent waters, Geng said.

"We strongly urge the relevant country to respect China's sovereignty and interest, and withdraw its personnel and facilities," Geng said.

He urged the relevant country to abide by bilateral and regional consensus, and not to take any action that may complicate the situation.