A top US diplomat has called on China to clarify or adjust its territorial claims in the South China Sea in accordance with international law.
Daniel Russel, assistant secretary of state for East Asia, criticised Beijing's so-called "nine-dash line" that outlines its claims.
He said there were "growing concerns" over China's "pattern of behaviour".
Tensions are already high over China's imposition of an air defence zone above disputed islands in the East China Sea.
Correspondents say there are fears of a fresh showdown in the South China Sea. Several countries claim competing sovereignty over islands, reefs and shoals.
China, the Philippines, Brunei, Malaysia, Vietnam and Taiwan all have claims in the region.
"There are growing concerns that this pattern of behaviour in the South China Sea reflects incremental effort by China to assert control over the area... despite objections of its neighbours," Russel told a congressional committee.
"Any Chinese claim to maritime rights not based on claimed land features would be inconsistent with international law," he said.
"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 added.
The US says it does not take stances on territorial disputes in Asia.
However, Russel said he supported the Philippines' right to take its case to a UN tribunal as part of efforts to find a "peaceful, non-coercive" solution.
China denounced the move last year.
On Wednesday, China's state news agency branded Philippine President Benigno Aquino a "disgrace" for comments in connection with the territorial row in which he compared China to Nazi Germany.
Aquino called for world leaders not to appease China over its claims in the South China Sea in the same way nations tried to appease Hitler before World War Two.
"At what point do you say: 'Enough is enough'? Well, the world has to say it. Remember that the Sudetenland [part of what was then called Czechoslovakia] was given in an attempt to appease Hitler to prevent World War Two," Aquino said in an interview with the New York Times on Tuesday.
An angry commentary on the state-run Xinhua news agency branded Mr Aquino an "amateurish politician who was ignorant both of history and reality".
China's "nine-dash line" stretches hundreds of miles south and east from its most southerly province of Hainan.
In January, Hainan province enacted new regulations requiring foreign fishing vessels to ask for permission to enter its waters, including the disputed areas claimed by China.
The Philippines said it was "gravely concerned" by the new rules, while Taiwan and Vietnam also said they did not recognise the rules.
Beijing says its rights come from 2,000 years of history where the Paracel and Spratly island chains were regarded as part of the Chinese nation.
More than half the world's merchant goods are shipped through the South China Sea and in 2010, then US Secretary of State Hillary Clinton declared that freedom of navigation there was a US national interest.
Relations between China and Japan are currently under strain over a separate territorial row involving islands in the East China Sea known as the Senkaku islands in Japan and the Diaoyu islands in China.
Last year, China announced an air defence identification zone (ADIZ) in the East China Sea, and said that aircraft flying through the zone must follow its rules, including filing flight plans.
The ADIZ covers the disputed islands, which Taiwan also claims, as well as a rock claimed by South Korea.
The US, Japan and South Korea have rejected China's zone, and flown undeclared military aircraft through it. The US has called the move a unilateral attempt to change the status quo in the region.