South Korea has protested to Japan over a tsunami advisory issued by its neighbour following the powerful New Year's Day earthquake that featured a map displaying a group of its islands also claimed by Tokyo, officials in Seoul said on Tuesday.

Although ties have improved in recent years, the two remain at odds over the sovereignty of the islets, called Dokdo in South Korea and Takeshima in Japan, which lie about halfway between them in the Sea of Japan, also known as the East Sea.

South Korea offered condolences for Monday's disaster, but added that the islands, shown on the map issued by Japan's weather agency, were not subject to any territorial dispute.

"Our government has sternly protested to Japan through a diplomatic channel and requested corrective action," Lim Soo-suk, a spokesperson of the South Korean foreign ministry, told a briefing.

Earlier on Tuesday, President Yoon Suk Yeol's office said he had sent a letter to Japanese Prime Minister Fumio Kishida, offering condolences for the victims and families as well as support for recovery efforts.

At least 48 people were killed after the quake, with a preliminary magnitude of 7.6, and subsequent tsunami waves, hit Japan's west coast.

On Monday, rescuers were struggling to reach isolated areas where buildings had been toppled, roads wrecked and power cut.

The territorial row was one of the lingering items of contention in the neighbours' ties, along with the mobilisation of forced labour at companies and of women in wartime brothels during Japan's 1910-45 colonisation of the Korean peninsula.

Japan also protested after annual drills around the islets last month by South Korea's military. Seoul's defence ministry responded that the exercises were regular training efforts meant to protect territory, citizens and assets.

South Korea stations a small band of coast guards on the islets.