The United States on Wednesday urged Beijing to clarify or adjust its claims in the South China Sea, calling for a peaceful solution to one of Asia's growing flashpoints.

With tensions already high over Beijing's imposition of an air zone above islands administered by Japan in the East China Sea, fears are growing of a fresh showdown in a separate row in the South China Sea where the Philippines is especially concerned.

Addressing the rifts, the top US diplomat for the region challenged Beijing's so-called "nine-dash line" that outlines its territorial claims over much of the South China Sea.

Danny Russel, the US assistant secretary of state for East Asian and Pacific affairs, said 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.

"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," he said.

Russel supported the Philippines' right to take its case to a United Nations tribunal -- a move last year that was denounced by China -- as part of efforts to find a "peaceful, non-coercive" solution.

"China's lack of clarity with regard to its South China Sea claims has created uncertainty in the region and limits the prospect for achieving mutually agreeable resolution or equitable joint development arrangements," Russel said.

Russel's remarks indicate an increasingly activist 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 United States, 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.

Warning against a new air zone

Concerns have been rising over the South China Sea. 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.

Japan's Asahi Shimbun recently reported that China has drafted proposals for an Air Defense Identification Zone over the South China Sea, similar to a move in November over the East China Sea that sent tensions soaring with Tokyo.

China denied the report, accusing "right-wing forces" in Japan of playing up tensions. The state-run Xinhua news agency separately attacked Aquino, saying his remarks showed him to be an "amateurish politician who was ignorant both of history and reality."

Russel repeated warnings to China not to impose an Air Defense Identification Zone, 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."

The United States, Japan and South Korea have defied China's zone in the East China Sea by flying fighter-jets through it without informing Beijing.

But Representative Steve Chabot, the chairman of the House Foreign Affairs subcommittee on Asia and a member of the rival Republican Party, accused President Barack Obama's administration of sending "mixed signals" that have emboldened China.

"It's time for the administration to move beyond speeches and find a way to reassure the region that the United States is there to stay, and that America's future in Asia is strong, committed and absolute," Chabot said.