North Korean leader Kim Jong-Un has ordered further nuclear tests.

North Korean leader Kim Jong-Un has ordered further nuclear tests, state media said Friday, as military tensions surge on the Korean peninsula with South Korean and US forces engaged in large-scale joint exercises condemned by Pyongyang.

Since the joint drills began Monday, the North has issued daily warnings and statements, talking up its nuclear strike capabilities and threatening to turn Seoul and Washington into "flames and ashes."

Just days after he was photographed posing in front of what state media described as a miniaturized nuclear warhead, Kim said the weapon required further testing.

Overseeing a ballistic missile launch on Thursday, Kim ordered "more nuclear explosion tests to estimate the destructive power of the newly produced nuclear warheads," the North's official KCNA news agency said.

The UN Security Council responded to the North's latest nuclear test and rocket launch by adopting tough, new sanctions, which Pyongyang condemned as a "gangster-like" provocation orchestrated by the United States.

Reacting to Kim's call for more nuclear tests, South Korea on Friday said the North Korean leader was being "rash" and displaying his ignorance of international opinion.

"The international community is imposing strong and comprehensive sanctions and this only goes to prove why they are necessary," said Unification Ministry Spokesperson Jeong Joon-Hee.

UN Secretary-General Ban Ki-moon on Thursday voiced grave concern over the growing tensions, and urged North Korea to avoid any further "destabilizing acts."

Kim, however, chose to highlight the need for a diversified nuclear strike force, capable of delivering warheads from the ground, air, sea and underwater.

The North has conducted a number of what is says were successful tests of a submarine launched ballistic missile.