The commander of US forces in the Pacific on Thursday accused China of "aggressive" behavior after a showdown between the two nations' navies.

The US military accused Chinese vessels of harassing the Impeccable ship in international waters of the South China Sea earlier this month. Beijing said the surveillance ship was engaged in illegal activities.

Admiral Timothy Keating, head of the US Pacific Command, said the incident showed that China's "behavior as a responsible stakeholder has yet to be consistently demonstrated."

"The Impeccable incident is certainly a troubling indicator that China, particularly in the South China Sea, is behaving in an aggressive, troublesome manner and are not willing to abide by acceptable standards of behavior or rules of the road," he told a Senate hearing.

But Keating said that US military relations with China have been improving and welcomed an easing of tensions between Beijing and Taipei since Taiwan elected Beijing-friendly President Ma Ying-jeou last year.

The naval showdown came days before Chinese Foreign Minister Yang Jiechi visited Washington, where he voiced hope for a strong relationship with President Barack Obama's administration.