The sinking of the Cheonan has remained a politicised issue in South Korea, where some have questioned whether Pyongyang really downed the ship.

An international investigation concluded that the vessel was torpedoed by a North Korean submarine, but Pyongyang has denied responsibility.

Seoul's hawkish President Yoon Suk Yeol, who took office last year, has shown public support for the victims of the Cheonan incident by wearing a T-shirt and hat that featured the ship.

The new Cheonan -- a 2,800-ton frigate with significantly enhanced anti-submarine capability -- began a "full-fledged mission to protect the West Sea" on Saturday, Seoul's navy said in a statement.

"We will be fully prepared for battle and equipped with unbeatable combat readiness posture," the Cheonan's chief said, according to the statement.

"If the enemy provokes in the West Sea, we will respond immediately and strongly until the end," the chief added.

The new Cheonan is equipped with long-range anti-submarine torpedoes and tactical ship-to-ship guided missiles, which make it capable of "attacking enemy submarines from long distances", the navy said.

The frigate also has the capability to "directly strike onshore facilities that command enemy submarines", it added.

The announcement comes as North Korea ramps up its nuclear and military threats, with leader Kim Jong Un saying Pyongyang will not hesitate to launch a nuclear attack if "provoked" with nukes.

North Korea on Monday tested its solid-fuel Hwasong-18 intercontinental ballistic missile, the largest weapon in its arsenal.

Pyongyang's launch last month of a military spy satellite, which it claimed quickly began providing images of US and South Korean military sites, further damaged inter-Korean ties.

The launch fractured a military agreement between the Koreas established to de-escalate tensions on the peninsula, with both sides then ramping up security along the Demilitarized Zone (DMZ) that separates them.