China plans to conduct a detailed survey of islands at the heart of an increasingly tense territorial dispute with Japan, state media said Tuesday.

The geographical study of islands in the East China Sea, as well as disputed territories in the South China Sea, is part of a comprehensive survey to map China’s “territorial islands and reefs” and safeguard its “maritime rights and interests”, the official Xinhua news agency said, without saying when it would take place.

State media did not elaborate whether the survey would involve activities on land or purely sea-based but acknowledged “difficulties” it faces.

“There are some difficulties in landing on some islands to survey, and in surveying and mapping the surrounding sea area of the islands, because some countries infringed and occupied these islands of China,” Zhang Huifeng of China’s National Administration of Surveying, Mapping and Geoinformation was quoted as saying.

The move underlines Beijing’s growing assertiveness in laying sovereignty claims and is likely aimed at dismissing Tokyo’s control over the Senkaku, or Diaoyu in Chinese, AFP reports.

Tensions have mounted since Tokyo purchased three islands from the chain, igniting anti-Japanese protests across China which has strained bilateral ties.

Chinese patrol vessels were also seen near the islands. Last month, Japanese fighter jets scrambled to the Senkakus after a lone Chinese state aircraft flew over the disputed islands’ airspace, Japan’s defence ministry said.