The US is obliged to protect those disputed islands in an event of a Chinese invasion under the mutual defence treaty dates back to the 1950s and it has been confirmed by the State Department that America recognises that those islands are under Japanese administration and it opposes any Chinese attempts to undermine it and China is unlikely to take any provocative moves that could lead to war as long as America maintains this stance to keep China in check.
As I’ve said many times before, although the United States does not take a position on the ultimate sovereignty of the islands, we acknowledge they are under the administration of Japan and we oppose any unilateral actions that would seek to undermine Japanese administration and we urge all parties to take steps to prevent incidents and manage disagreements through peaceful means.
http://www.state.gov/secretary/rm/2013/01/203050.htm