South Korea, the US and Japan have conducted their first joint missile-tracking drill, in the waters off the US state of Hawaii.
The move follows North Korea’s repeated tests in recent months of mid-range ballistic missiles.
Most tests have ended in failure, but the apparent success of the sixth last week alarmed the region.
North Korea, which has also conducted four nuclear weapon tests, said the drills were “military provocation”.
State media said the US and other “hostile forces” were a “constant threat” to North Korean security and reinforced its commitment to pursuing ballistic and nuclear weapons.
The US military said the drills, called Pacific Dragon, would enhance the “already strong relationship of all three nations participating”.
No missiles were fired, said the US Third Fleet, but each country tested its Aegis Ballistic Missile Defense System and tested communications and data collection.
The Aegis system allows warships to shoot down enemy ballistic missiles while they are still in space, before there is any danger of causing any damage.
North Korea is banned by UN resolutions from any use of ballistic missile technology.