Beijing's claim to almost the whole of the South China Sea is "problematic" and the Asian giant's actions have raised tensions, a senior US official said Tuesday on the eve of high-stakes talks.
China also disputes islands with Japan -- a US security ally that Washington is treaty-bound to defend if attacked -- and officials travelling with Secretary of State John Kerry said they had "heightened concerns" about "the readiness of claimants to utilize military, paramilitary, coastguard forces in furtherance of their claims".
Kerry arrived in Beijing Tuesday for the sixth Strategic and Economic Dialogue, billed as the main annual meeting between the world's two largest economies.
The two days of tough discussions will seek to chart a path ahead in turbulent China-US ties roiled by differences over Asia-Pacific maritime tensions, Internet hacking and trade issues.
China's claim to the strategic South China Sea -- also claimed in part by the Philippines and Vietnam, among others -- is based on a line drawn on 20th century maps.
"The ambiguity associated with the nine-dash line is problematic," said a US official travelling with Kerry.
China and its neighbors have stepped up their patrols of disputed areas, and recent spats in the South China Sea have led to boats ramming each other, the use of water cannon and arrests of fishermen.
The ambiguity associated with the nine-dash line is problematic," -- a US official said as he is accompanying with Secretary of State John Kerry for the two-day Strategic and Economic Dialogue in Beijing
The heightened tensions are "very relevant to the United States as a Pacific power, as a major trading nation, as an important consumer of the sea lanes and as a long-term guarantor of stability in the Asia-Pacific region," the senior US administration official said.
The official spoke anonymously in order to discuss the talks frankly, and added that the conversation will be held in "a very direct, candid, and constructive way".
The US stresses that it takes no sides in the territorial claims, but has accused Beijing of destabilizing acts and urged it to uphold freedom of navigation in the key waterways.
China has said it is committed to diplomatic, peaceful means to address the claims, the US official said. "We want China to honor that and live up to its word."