From harassing Filipino fishing boats and monitoring oil exploration off Vietnam to playing cat-and-mouse with the Japan Coast Guard, China's expanding fleet of civilian patrol vessels have become the enforcers in disputed Asian waters.

The ships of the recently unified Chinese coast guard are a fixture around the disputed islands and shoals of the South and East China seas. While the ships don't have the weaponry of military vessels, thus reducing the risk a confrontation could get out of control, they still represent a potent show of sovereignty.

The coast guard is funded by China's State Oceanic Administration, a civilian body, although one U.S. naval officer and security experts said it coordinates its operations with the People's Liberation Army (PLA). It includes the 200-strong China Marine Surveillance fleet and is seen as another example of how hard it is to get a true picture of China's defense-related spending.