The US voiced concern that China is gradually asserting control in the disputed South China Sea on the basis of vague territorial claims unsupported by international law.
Sharpening criticism of the rising Asian power, Daniel Russel, the senior US diplomat for East Asia, described in congressional testimony a litany of actions by China that are raising tensions.
He said China has restricted access to a contested reef in the South China Sea, taken bids on areas far from its own shores for hydrocarbon exploration and imposed fishing regulations in disputed waters.
"There is a growing concern that this pattern of behaviour in the South China Sea reflects an incremental effort by China to assert control over the area contained in the so-called 'nine-dash line,' despite the objections of its neighbours," he said, referring to the map markings used by China to depict its maritime territorial claims.
"Any use of the 'nine-dash line' by China to claim maritime rights not based on claimed land features would be inconsistent with international law," he added, urging China to clarify or adjust its claim.
China cites a historical basis for its South China Sea claims. According to the US Congressional Research Service, the nine-dash line covers roughly 80% of those resource-rich waters, which are dotted with reefs and islands subject to multiple disputes, also involving the Philippines, Vietnam, Malaysia, Brunei and Taiwan.
The US says it takes no position on the competing sovereignty claims, but that it has a national interest in peaceful resolution of the disputes and in freedom of navigation and commerce.
Mr Russel said agreement between China and Southeast Asia's regional bloc on a code of conduct to regulate behaviour in the South China Sea is long overdue.
China has been reluctant to discuss territorial disputes with the bloc, although there has been tentative progress on negotiating the code in the past year.
Mr Russel also voiced concern about the "serious downturn" in relations between China and Japan, a close US ally.
He criticised China's "provocative" declaration of an air defence identification zone over the East China Sea and a spike in "risky activity" by China near uninhabited islands controlled by Japan but also claimed by China.
He said the US supported Japan's call for diplomacy and crisis management procedures to avoid a dangerous incident.
Japan's foreign minister Fumio Kishida will meet Secretary of State John Kerry in Washington tomorrow.
Japan's prime minister recently angered China and drew a rare US expression of disapproval for visiting a controversial war shrine in Tokyo.