Tension is building between South Korea and Japan after Seoul sent a ship to survey waters around a group of islands claimed by both countries.
The islands are small, but highly symbolic in S Korea-Japan relations
Japanese Chief Cabinet Secretary Shinzo Abe asked South Korea on Monday to "refrain from carrying out the survey".
In April, the two nations staged a war of words after Japan announced its own plans to survey the area.
The islands, called Dokdo in South Korea and Takeshima in Japan, sit in rich fishing grounds.
They may also contain extensive deposits of gas.
The islands are currently under South Korean control, and Seoul maintains a police presence there.
Studying the sea
The South Korean ship left port on Sunday night, with about 20 people aboard.
It is scheduled to conduct its survey in mid-July, although Kim Ok-Soo, a chief survey planner, said the date the ship actually enters disputed territory will depend on weather conditions.
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
South Korea's National Oceanographic Research Institute, which organised the survey, told the French news agency AFP that the ship would study sea currents, temperatures and saline density.
"It is the basic right and prerogative of this country to conduct any scientific research within our exclusive economic zone area," a South Korean government official told Reuters news agency.
But Japan has called on South Korea to cancel the trip.
Tokyo's top government spokesman, Shinzo Abe, told reporters on Monday that he had not yet received official confirmation that Seoul had decided to survey the area, but asked the South Koreans to refrain from conducting the exercise.
Should Seoul choose to go ahead, though, Tokyo would "respond appropriately," Mr Abe said.
The tiny islands are about midway between Japan and South Korea.
Tensions over them are fuelled by lingering historical acrimony.
Both countries have centuries-old claims to the islands, which were annexed by Japan in 1905, five years before it annexed the Korean mainland. After World War II, South Korean coastguards took control of the islands in 1953.
South Korea accuses Japan of failing to repent for its colonial conduct, and the rivalry between the two nations repeatedly flares into confrontation.
The last such argument happened in April, when Japan announced it would carry out a survey of the area.
Tokyo shelved the idea after loud protests from South Korea.