The US on Wednesday urged Beijing to clarify or adjust its claims in the South China Sea, a move denounced by Chinese observers as interference in China's internal affairs.

Danny Russel, the US assistant secretary of state for East Asian and Pacific affairs, challenged Beijing's nine-dash line, adding that maritime claims under international law needed to be based on land features.

"Any Chinese claim to maritime rights not based on claimed land features would be inconsistent with international law," Russel told a congressional committee.

Russel supported the Philippines' right to take its case to a United Nations tribunal, a move denounced by China.

"China could highlight its respect for international law by clarifying or adjusting its claim to bring it into accordance with international law of the sea," Russel said.

Shi Yinhong, director of the Center on American Studies at the Renmin University of China, told the Global Times that Russel's comments were to make clear the bottom line of the US in issues regarding the South China Sea and to deter China.

"The US sees the South China Sea as an important strategic channel and believes it will play a key role in China's expansion, which will lead to future contest with the US," Shi said.

Russel's remarks indicate an increasingly active US stance on the South China Sea. In 2010, then secretary of state Hillary Clinton declared on a visit to Vietnam that freedom of navigation was a US national interest in the South China Sea, through which more than half of the world's merchant goods are shipped.

But the US, while boosting military cooperation with allies Japan and the Philippines, has generally stressed that it takes no stance on sovereignty in Asia's myriad disputes, a position that Russel reiterated.

Russel also repeated warnings to China not to impose an Air Defense Identification Zone (ADIZ), which calls on planes to report to Beijing, over the South China Sea.

"We neither recognize nor accept China's declared ADIZ," Russel said. "We made clear to China that it shouldn't attempt to implement that ADIZ and should refrain from taking similar actions elsewhere in the region."

Ji Qiufeng, a professor of international relations at China's Nanjing University, told the Global Times on Thursday that Russel's comments came out of a cold war mentality which sees China as a potential threat and it reflected the US intention to contain China under the "pivot to Asia" or "rebalancing" strategy.

"The US turned to South China Sea recently as it is now hard to harmonize Japan and South Korea in the US-Japan-South Korea triangle. Its doubts over China's sovereignty could be regarded as interference in our internal affairs," he said.

Meanwhile Philippine President Benigno Aquino in an interview with The New York Times called on world leaders not to "appease" China and drew a parallel to the 1938 decision to give Czechoslovakia's Sudetenland to Adolf Hitler's Germany.

His comment was slammed by Xinhua, which said the comments showed Aquino was an amateurish politician who was ignorant of both history and reality.