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 utilise 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 neighbours 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 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 destabilising 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 honour that and live up to its word."
- North Korea, cyber-attacks on agenda -
Other issues high on the agenda include nuclear-armed North Korea, following a "significant" visit last week by Chinese President Xi Jinping to Seoul.
"There is a steady convergence in the views between the US and China on both the importance and the urgency in moving North Korea to take irreversible steps to denuclearise," the official said.
But while Beijing's patience with the brinkmanship of its wayward, unpredictable ally appears to be wearing thin, it has not publicly shown any willingness to take any action towards the regime.
Kerry is to meet with Xi on Thursday.
Chen Zeguang, China's assistant foreign minister, said Monday that the US "has made some wrong remarks and acts on maritime issues and cyber issues, bringing negative impact on bilateral relations."
"We are committed to working with the US side to advance such a relationship," Chen told reporters in Beijing. "To achieve this objective, the most important thing is that we should not see confrontation or conflicts."
Despite the Obama administration's "pivot" to Asia, already fraught US-China relations have strained further since Washington in May indicted five Chinese military officers for hacking into US businesses.
"Chinese actors... stole corporate or proprietary information... and transferred it to state-owned enterprises for the purpose of commercialising it," the US official alleged.
Although analysts saw the charges as a largely symbolic warning to Beijing since the men are unlikely to appear in a US court, China angrily cancelled key cybersecurity talks due this week.
Kerry, accompanied by Treasury Secretary Jacob Lew and a large US team, will seek to persuade their Chinese counterparts, led by State Councilor Yang Jiechi and Vice Premier Wang Yang, to reinstate the working group to draw up rules for using and protecting the Internet.
As arguably the two largest users of cyberspace "we share an interest in a secure, predictable and orderly cyber environment," the US official said.
Chen on Monday dismissed the charges against the five Chinese officers as "intentionally fabricated by the United States".
While US officials said it was hard to predict if there would be any dramatic breakthroughs in the talks, they lauded the dialogue as a mechanism for keeping "deep, broad and very resilient" China-US ties on track.
"The US-China relationship is a motion picture, it shouldn't be looked at as snapshot," said a second US official.
"It's sort of that grand epic big Hollywood motion picture in which there's a lot of actors, a lot of interests at stake, and the trajectory of any particular issue takes time to play out."