South Korea, the US and Japan held joint missile defense drills off the Korean Peninsula on Tuesday to counter North Korea’s aggression, as the North vowed to expand its capability to use nuclear weapons at sea.

The exercises in international waters off Jeju Island focused on tracking projectiles and sharing information about such potential launches by North Korea. Destroyers equipped with the Aegis system to shoot down missiles were used, South Korea’s Navy said.

The aim is to “enhance readiness for North Korea’s growing nuclear and missile threats,” it added.

The drills, the first joint exercises since the three countries pledged tighter security ties at their landmark Camp David summit on Aug. 18, came two days after the North Korean leader vowed to expand using nuclear weapons at sea.

In a speech marking the North’s Navy Day, leader Kim Jong-un called for a stronger navy to counter what he called “gang bosses,” the state-run Korean Central News Agency reported Tuesday, referring to the three leaders who met at the US presidential retreat.

There, the three nations agreed to share real-time data on Pyongyang’s missile launches and annually hold military drills to bolster readiness. The first standalone gathering doubled down on a nuclear-armed North Korea and expansionist China, the North’s biggest supporter.

At Navy Day celebrations, Kim posed for pictures with his daughter Ju-ae, a signal analysts say is meant to sanctify Kim’s bloodline.

“Kim wants to say that the navy and his daughter will be the guardian against threats outside and that they together will be with next-generation North Koreans,” said Yang Moo-jin, a professor at the University of North Korean Studies in Seoul.

Kim’s push for strong naval forces means Pyongyang is expecting the sea “to be the place” for the South, the US and Japan to best deepen their security cooperation, Yang said. “And Kim wants to show off that he still has the initiative, the military upper hand over the Korean Peninsula,” he added.

Last week, North Korea failed for the second time to put a spy satellite into orbit over rocket troubles, an attempt the leader has said will take place again soon. The isolated country -- banned from using ballistic missile technology which is also used in satellite launches -- wants to track US forces stationed in the region.

Currently, Seoul and Washington are running separate joint military drills, which end at the end of this month. Pyongyang, which has warned of nuclear war, has called them a rehearsal for invasion. The two allies have said that they are meant to defend against outside attacks.