Taiwan has accused Japan of "furtively occupying" disputed islands in the East China Sea which are at the centre of a territorial row between Tokyo, Beijing and Taipei.

The move came as Japan, which controls the islands, deported a group of pro-China activists who had sailed to one of them, and could be a sign that Taipei will take a stronger stance on the issue, Taiwanese media said.

On Wednesday, some of the activists became the first non-Japanese to set foot on any part of the archipelago since 2004, planting China's and Taiwan's flags there.

"It is in keeping with our government's territorial claim that the national flag of the Republic of China (Taiwan's official name) was on the Diaoyu islands, which Japan is furtively occupying," the foreign ministry said in a statement late Friday.

The islands are known as Diaoyu in China and Taiwan and Senkaku in Japan.

It was the first time Taiwan has described Japan as "furtively occupying" them.

The activist group, from the Action Committee for Defending the Diaoyu Islands, has said its journey was aimed at countering a plan by a group of Japanese lawmakers to visit the chain this weekend.

A group of Taiwanese activists hoping to join the trip had to abort their plan after failing to hire a ship.

Japan, like most countries, officially recognises Beijing rather than Taipei but has maintained close trade and cultural ties with Taiwan, which was its colony between 1895 and 1945.