Maritime Self-Defense Force (MSDF) vessels have refueled U.S. Navy Aegis ships in the Sea of Japan and surrounding areas since May under revised Japan-U.S. security arrangements, government sources said Sept. 14.

The refueling work, often carried out simultaneously on a number of ships, is a continuous activity because the U.S. warships are watching out for North Korean ballistic missile launches, a constant concern, the sources said.

The MSDF’s refueling activities became possible in April, when revisions of the Acquisition and Cross-Serving Agreement (ACSA) between Japan and the United States took effect following the enforcement of new national security legislation in March 2016.

Under the ACSA concluded in 1996, the SDF and the U.S. military could cooperate in transportation as well as in sharing food and fuel. But these activities were limited to joint exercises and certain other cases.

U.S. ships in the Sea of Japan that were monitoring North Korea’s actions had to leave the area for refueling under the previous agreement.

The U.S. government had urged Japan to include the refueling of U.S. ships checking for North Korean missiles in activities listed in the ACSA.

The range of possible SDF activities stipulated in the ACSA gradually expanded.

Now, U.S. warships do not have to leave the area because they can be refueled by MSDF vessels.

The national security legislation passed the Diet in 2015, despite concerns that it raises the risk of Japan becoming embroiled in a war overseas. The ACSA was revised in September 2016 and took effect in April.