North Korea has pledged to carry out "merciless retaliation" on the South Korean and American "warmongers" who are holding a joint miliary exercise.

Pyongyang's latest threats have come after a series of similar warnings against the ongoing drill jointly conducted by Washington and Seoul.

The Ministry of the People's Armed Forces in Pyongyang has released a statement via the official Korean Central News Agency, saying: "Warmongers would be well advised to keep in mind that the DPRK is no longer restrained.

"What is left to be done is an action of justice and merciless retaliation of the army and people of the DPRK."

The threat has come a day after the North Korean leader Kim Jong-un visited the strategically important military installation on the western coast. According to the state-run meadia, he ordered the troops to "drive the enemy into a fire pit if so ordered."

Kim was accompanied by top military officials including the army chief and the minister of armed forces.

The computer-simulated drills between South Korea and the US, codenamed 'Key Resolve', began on 11 March are likely to continue until 21 March. Nearly 10,000 South Korean troops and 3,500 American forces along with fighter planes are involved in the manoeuvres.

North Korea's statement once again insisted that the defiant nation will continue to pursue its nuclear programmes in order to defend its territories.

Reports from Seoul also suggest North Korea has significantly increased the flights of its fighter jets over past few days coinciding with the military exercise.

According to the Yonhap news agency report, North has made as many as 700 flights on the starting day of the drill.

Meanwhile, US intelligence chief James Clapper has also expressed concerns that the North has taken steps to deploy mobile long-range intercontinental ballistic missiles (ICBM).

"Last April it displayed what appears to be a rogue mobile intercontinental ballistic missile. We believe North Korea has already taken initial steps towards fielding this system, although it remains untested," Clapper, the Director of National Intelligence, told a senate hearing.