The United States wants China and rival countries to voluntarily halt actions aggravating territorial disputes in the South China Sea.
US deputy assistant secretary of state Michael Fuchs said no country was solely responsible for escalating tensions in the region. But he reiterated the US view that "provocative and unilateral" behaviour by China had raised questions about its willingness to abide by international law.
Washington wanted the 10-member Association of Southeast Asian Nations (Asean) and China to have "a real and substantive discussion" to flesh out a call for self-restraint contained in a Declaration of Conduct (DOC) they agreed to in 2002, with a view to signing a formal maritime Code of Conduct, Fuchs said.
"We've called for claimant states to clarify and agree to voluntarily freeze certain actions and activities that escalate disputes and cause instability as described in the DOC," Fuchs told a Washington think tank, giving details of a proposal he said had been put to both China and Asean states, but which has received little public attention.
A US official said the issue was raised again last week with China at the annual strategic and economic dialogue. He declined to give details of China's response.
Fuchs said deciding what elements were included in a freeze would ultimately be up to the claimants, but these could include recommitting not to establish new outposts or seize territory another claimant had occupied before the 2002 declaration.
Claimants could also clarify what types of action were provocative and which merely constituted efforts to maintain a long-existing presence dating to before 2002, Fuchs added.
Fuchs said claimants could also agree to refrain from unilateral enforcement measures against long-standing economic activities by other claimants in disputed areas.
"The agreement would not affect any party until all claimants had agreed to abide by its terms," he said.
Ni Lexiong, a Shanghai-based military-affairs commentator, said the remarks by Fuchs indicated that Washington was "worried about the expansion of China", but Beijing would probably ignore his remarks.
China is reportedly building outposts around the disputed waters, and looking to expand its biggest installation in the Spratly Islands.
"China believes all the infrastructure is for its own development," he said.
Additional reporting by Teddy Ng