East China Sea - Japanese Coast Guard planes made seven flights over Chinese marine surveillance ships in the past week, ever since two Chinese patrol ships expelled two Japan Coast Guard survey vessels during a routine patrol in the East China Sea.

The two Chinese ships patrol waters near gas fields of Chunxiao and Pinghu, Suyan Rock and China-Japan joint development zones.

On Feb 19, China Marine Surveillance Ship 66 detected a Japanese survey ship SHOYO and a Japanese Coast Guard vessel TAKUYO traveling at a low speed in the waters off Chunxiao gas field.

An official on the Chinese patrol ship told China Daily that a?speed of less than 9.3 kilometers per hour is an indicator of whether a ship is carrying on surveying and investigation activities.

China Marine Surveillance Ship 66 came within 550 meters of the Japanese survey ship SHOYO and demanded the ship stop its illegal investigation in China's jurisdictional waters.

On the morning of Feb 20, the two Japanese ships left the area.

China has carried out routine patrols in the East China Sea since 2006 on a monthly basis, according to a spokesman from China Marine Surveillance (CMS), China's offshore law enforcement agency.