Government official believe members of rightwing Japanese groups will attempt to land on Dokdo, angered by President Lee Myung-bak's recent visit to Korea's easternmost islets. Japan maintains a dubious claim to the islets.

"The chances" that they will storm Dokdo "have become stronger than ever," a government official here said on Thursday.

Japanese extremists could be inspired by the recent landing of Chinese activists on the Senkaku or Diaoyu Islands, which are disputed between China and Japan.

The Dokdo islets and their only port (in a dotted circle)

The government apparently set up a response plan in 2005 to thwart such an attempt and has conducted exercises every year based on six different scenarios.

Korean maritime police have stationed 3,000 to 5,000-ton patrol vessels near Dokdo, which are capable of blocking even large ships. But maritime police do not rule out that extremists could somehow reach the islets.

Dokdo is surrounded by around 90 rock formations including two islands -- east and west. The only port is located on the east side and it is difficult to reach when waves are high.

"There are also a lot of underwater reefs making it difficult to land there," a maritime police official said.