Hua urged the US to clarify whether the freedom of navigation it claims is the freedom of navigation that abides by international law or simply navigation enjoyed solely by the US military.

"If it is the latter, I'm afraid that the whole international community will not agree."

Zhu Feng, head of a South China Sea research center at Nanjing University, said Washington is practicing gunboat diplomacy.

"China never challenges the freedom of navigation of civil or commercial vessels in the South China Sea," Zhu said.

Obama, who started a three-day visit to Vietnam on Monday, has announced the lifting of a decades-old ban on weapons sales.

Tom Pepinsky, a Southeast Asia expert and associate professor of government at Cornell University, said, "This decision signals the US plans to contain China's regional ambitions, with Vietnam as a partner."