Officials from Tokyo’s metropolitan government surveyed the disputed islands off the East China Sea in a symbolic move to assert its claims.

Experts and journalists, along with Tokyo city officials, boarded a boat to survey the five uninhabited islands but Japan’s central government prohibited the boat from landing on the islands. The visit lasted 10 hours, enough time to measure the water’s depth. Officials told Asahi Shimbun that they plan to build a dock at the islands following their¬†purchase. China quickly responded, saying that Japan’s unilateral action was “illegal” and “invalid”.

The Senkaku Islands, or Diaoyu in Chinese, remain a diplomatic flashpoint between China-Japan relations. Several activists from Hong Kong and Japan have landed on the islands to defend claims, setting off protests from both sides.