The US navy says the Chinese navy has been less aggressive in contested waters in the Asia-Pacific region.
The head of the US Pacific Command, Admiral Robert Willard, said this was a "positive" development.
He linked the apparent change in approach to strong statements from US officials against Chinese actions.
China's growing military strength has sparked several clashes in recent years in East Asian waters where it claims exclusive rights.
"There has been a retrenchment a bit by the Chinese navy, such that while we continue to experience their shadowing of some of our ships that are operating in some of these waters, we have not seen the same level of assertiveness in 2011 that we witnessed in 2010," Adm Willard told the Senate Armed Services Committee.
He believed that the resumption of military talks between the US and China has helped, and said "perhaps we can make an advancement in that regard".
US Secretary of State Hillary Clinton and Defense Secretary Robert Gates had made "very strong statements", he noted.
Mrs Clinton told a regional summit in Vietnam last year that the US supported non-Chinese claimants to the sea.
The admiral said that there was no doubt that China "aims to have great influence over that maritime space, and especially over the contested areas that they've laid claim to in both the South China Sea and the East China Sea."
Its building of a first aircraft carrier was part of that and would have a profound effect on the balance of forces in the region, he said.
The carrier is an old Soviet craft bought from the Ukraine over a decade ago which has been remodelled; it will not be operational until lengthy tests are completed.
"But I think as a symbol, the feedback that we receive in our dialogue throughout the region is that the regional partners regard this step by the Chinese in the midst of what has otherwise been a remarkable growth in their military capability as significant," he said.
The US Pacific Command has five aircraft carrier strike groups, which have formed a security umbrella under which most Asian states have been able to freely develop wealthy trading economies.