China has conducted aerial surveys several times this year beyond the "median line" with Japan in the East China Sea amid the ongoing bilateral gas exploration dispute, leading Tokyo to lodge complaints with Beijing through diplomatic channels, sources close to the matter said Saturday.

After Beijing notified Tokyo of the plan in advance, Japan urged China to provide information such as the purpose of the surveys, but the Chinese side carried them out without responding, the sources said.

They said the surveys were apparently part of China's exploration for natural resources.

Some analysts said they believe China was trying to see how Japan would respond to the moves at a time when bilateral ties remain strained due to the gas dispute and Prime Minister Junichiro Koizumi's visits to Yasukuni Shrine.