A U.S. naval aircraft carrier is not participating in Seoul, Washington and Tokyo's multi-domain "Freedom Edge" exercise this week, a drill meant to promote trilateral security cooperation against North Korea's security threats.

Seoul's military authorities announced Tuesday that the three countries had begun the five-day drills in international waters southeast of Jeju the previous day, bringing together their respective naval vessels and aviation powers.

The absence of a U.S. naval aircraft carrier in the latest round of exercise is considered unusual, especially as Washington places keeping Beijing in check as a priority in its defense policy.

A Seoul military source said the carrier's absence could stem from two carrier groups currently being deployed to the Middle East, but some have suggested that the decision could have been made in consideration of North Korea's protest against the trilateral exercise.

The training aims to improve defense against sea-launched missiles, anti-submarine operations, air defense, anti-piracy and digital defense, with the three nations' Aegis destroyers, frigates, fighter jets, patrol planes, early warning aircraft, aerial tankers and helicopters taking part.

The latest Freedom Edge exercise is the third of its kind. The inaugural and second regular drills took place in June and November of last year respectively.