North Korea has accused the United States of “openly ignoring” the North’s security concerns after a nuclear-powered U.S. submarine arrived at a naval base in South Korea.

In a statement carried by the North’s state Korean Central News Agency on Tuesday, the defense ministry warned that the North will “make clearer its choice of action and mode of counteraction against the U.S.”

The warning came a day after the USS Alexandria, a Los Angeles-class attack submarine, entered the naval base in Busan to replenish supplies and allow its crew to rest.

The North denounced the move as a clear expression of the United States’ hysteria for confrontation, a threat to the North’s security environment, and a factor in escalating the military tension in the region.

The North’s defense ministry expressed “grave concern” over what it called a “dangerous hostile military act” by the United States, saying the submarine’s presence could lead to an armed conflict and calling on the U.S. to stop its provocations.