The Japan Coast Guard and Taiwan's Coast Guard Administration carried out a joint maritime exercise earlier this week, south of the Boso Peninsula in Chiba Prefecture, Taiwanese government sources have said.

The two sides had conducted undisclosed joint drills Thursday based on a memorandum regarding maritime rescues, but this is the first time that such training has been made known to the public.

According to the sources, Taiwan's Hsun Hu No. 9 patrol ship engaged in a search, rescue and communication drill with the Japan Coast Guard's Sagami patrol vessel. Prior to the exercise, the Taiwanese vessel made a port call at Tokyo for supplies of fuel, water and food.

Tokyo and Taipei are on alert over China's increasingly coercive actions in the East and South China seas. Chinese coast guard ships have frequently entered Japanese waters off the Senkaku Islands, controlled by Japan but claimed by Beijing, and have stepped up patrol activities near Taiwan's Kinmen Islands.

Since signing the memorandum in 2017, Japan and Taiwan have cooperated in exercises for maritime rescues that could be conducted in waters close to both sides. The drills are believed to be kept under wraps so as not to excessively provoke China.

In Tokyo, Japanese Chief Cabinet Secretary Yoshimasa Hayashi told a news conference Friday that joint drills are not conducted "with a third-party country such as China in mind."

China said Friday that it firmly opposes Japan undertaking joint maritime training with Taiwan, the Foreign Ministry said, adding that it had lodged a complaint with Japan.

"We urge Japan to abide by the 'one-China' principle ... correct its mistakes immediately, not condone and support Taiwan independence separatist forces in any form," Lin Jian, a Foreign Ministry spokesperson, said at a regular news conference.