North Korea fired multiple cruise missiles off its east coast on Wednesday, as South Korea and the U.S. held joint military exercises, according to the South Korean military.

The test comes just days after North Korea said a missile it launched on Monday was fired as a simulated nuclear attack on South Korea, as North Korean leader Kim Jong Un continues to respond to what he sees as growing aggression from the South and its allies. 

The South Korean military said it could not confirm how many missiles were fired on Wednesday or what type they were, but said they were fired toward the Sea of Japan.

The “aggression” that the North Korean leader points to is the joint military exercises between the U.S. and its Asian allies, including South Korea and Japan. The U.S. has sought to strengthen those ties as tensions in the region flare.

The test came amid an 11-day joint military “Freedom Shield” exercise between the U.S. and South Korea that is slated to end on Thursday. The operation is the largest between the two countries in at least five years and it is one that North Korean officials see as preparation for a possible invasion.

The missile launches on Wednesday mark North Korea’s sixth missile tests just this month.

The aggression shown by the North Korean military follows a trend of unprecedented missile tests last year. U.S. intelligence agencies concluded earlier this month that North Korea had been timing its missile tests to coincide with joint military exercises between the U.S. and South Korea.