The Everett-based aircraft carrier USS Nimitz spent the weekend on a port call in Busan, South Korea. The trip came amid rising tensions between the United States and North Korea. See more photos from the port call in our gallery. On Monday, the ship headed to the Sea of Japan for joint drills with the South Korean navy, Stars and Stripes magazine reported. North Korea called the exercises "a grave military provocation to unleash a nuclear war," the magazine said.