South Korean and Japanese officials are holding talks to try to resolve an escalating row over disputed islands.
South Korean coastguards have been put on full alert
Seoul put its coastguard on alert after Tokyo announced plans to conduct a maritime survey of the islands known as Dokdo in Korea and Takeshima in Japan.
South Korean Foreign Minister Ban Ki-moon summoned Japan's ambassador in Seoul to protest against the plans.
Tokyo has indicated it will call off the survey if Seoul drops plans to rename seabeds around the islands.
Two Japanese coastguard vessels are reported to have left port, but have so far held back from entering the disputed waters, the BBC's Charles Scanlon in Seoul reports.
A flotilla of 20 South Korean patrol boats, backed up by helicopters and reconnaissance planes, are waiting to greet them.
Japan's plans have provoked an emotional nationalistic response in Seoul, our correspondent says.
President Roh Moo-hyun compared Japan's action to its former colonial aggression.
Known as Dokdo (Solitary islands) in Korea, Takeshima (Bamboo islands) in Japan
Also known as Liancourt rocks
Japan's and South Korea's claims go back centuries, but islands occupied by S Korea since 1953
Just 230,000 sq m in size, with no fresh water
But surrounding waters valuable for their fishing
"We are now in a difficult situation," he said. "Problems cannot be solved just by goodwill, and we need wisdom and courage."
Japan's chief government spokesman, Shinzo Abe, said Tokyo planned to carry out the survey "that is based on international law", but added there were "unofficial" contacts between both sides to seek a peaceful resolution.
Japanese media reported on Thursday that Tokyo had offered to drop its plans if South Korea agreed not to attempt to register Korean names on underwater basin areas around the islands.
The isolated rocks, which lie between South Korea and Japan, have no settled population, but have been occupied for decades by South Korean police.
The dispute over the islands is a recurring flashpoint in relations between the two countries, whose relations have soured in recent years.
South Korea has joined China in condemning visits by the Japanese prime minister to the controversial Yasukuni war shrine.
And it has accused the Japanese government of trying to whitewash its history of aggression in Asia.