Japan says it has strongly protested South Korea's maritime survey conducted near the Dokdo islets late last month.

In a regular briefing on Monday, Japan's deputy chief cabinet secretary, Kihara Seiji, said that Tokyo has confirmed that three South Korean research vessels sailed near the islets between July 24 and 28.

According to the Sankei Shimbun newspaper, the Japanese Coast Guard made radio contact with the ships but received the message that the maritime survey was a justified activity as Dokdo belongs to South Korea.

Japan, which claims the islets to be Japanese, has historically protested such surveys conducted by Seoul, designating the waters near Dokdo as part of its exclusive economic zone.

The most recent complaints came earlier this year when Japan asked the Korea Hydrographic and Oceanographic Agency to stop its regular surveys near Dokdo in May and June.

Seoul categorically dismissed Tokyo's protests, stating that Dokdo is Korean historically, geographically and under international law.