The Japanese Self-Defense Forces and the U.S. military named China as a hypothetical enemy for the first time in their joint command post exercise, government sources said Sunday, amid rising concerns over a potential invasion of Taiwan by Beijing in the future.

The computer simulation exercise, which began on Feb. 1 and is slated to be held through Thursday, envisions an emergency in Taiwan. A provisional name was previously used when referring to an enemy.

Japan’s Defense Ministry is believed to have classified the scenario as a specially designated secret under the country’s secrecy law