BEIJING - Observers of several renowned Chinese think tanks on Thursday backed the foreign ministry's note on the "fundamental changes" to the Diaoyu Islands situation.

Chinese Foreign Ministry spokesman Hong Lei said at a routine press briefing on Tuesday that "the Diaoyu Islands situation has gone through fundamental changes" and warned Japan not to have any illusions about related territorial claims.

"The judgement was well-backed and true," said Gong Li, head of the Institute of International Strategic Studies under the Party School of the Communist Party of China Central Committee.

Gong noted that the so-called "actual control" by Japan over the Diaoyu Island and its affiliated islets since 1971 have been destroyed, as the Chinese side has taken a series of resolute and forceful countermeasures to demonstrate its sovereignty and exercise control over the islands.

The Japanese government should understand the situation, admit that there is a dispute over the islands, give up illusions and correct its mistakes, which would be the only wise move, said Gong.

According to a statement issued by China's State Oceanic Administration (SOA), a fleet of four marine surveillance ships encountered Japanese vessels at 10 am on Tuesday on a routine patrol within the 12 nautical miles that constitute China's territorial waters off the Diaoyu Islands.

The Chinese fleet conducted surveillance on the Japanese vessels, took photographic evidence of their presence and expelled a number of Japanese vessels from the area, said the statement.

The operation was significant and an enhanced measure for safeguarding and exercising China's sovereignty over the Diaoyu Islands, according to Gao Hong, deputy director of the Institute of Japanese Studies, the Chinese Academy of Social Sciences.