South Korea's acting president Hwang Kyo-ahn has ordered the country's military to be placed on combat readiness to provide an "immediate response" in case there is any provocation from North Korea. Hwang warned Pyongyang could flex its military muscle any time.

The warning has come amid growing concerns that the Kim Jong-un regime may conduct either a nuclear test or test-fire an intercontinental ballistic missile (ICBM). The North has been holding several events since mid-April to commemorate key occasions including the 105th anniversary of the country's founding leader, Kim Il-sung.

"I call on the military to keep close tabs on the possibility of North Korea's provocations and strengthen its [readiness] posture for an immediate response," Hwang told a group of cabinet ministers on Thursday, 20 April.

The interim leader cited the recent botched missile launch to caution that North Korea could be gearing up to carry out a major provocative act "at any time".

Meanwhile, in a separate development, the US has revealed that North Korea could be blacklisted as a state sponsoring terrorism in a bid to put more pressure on the Kim regime. Speaking to reporters, US Secretary of State Rex Tillerson said Washington was actively examining the possibility of adding the North to the list.

"We're reviewing all the status of North Korea both in terms of state sponsorship of terrorism as well as all the other ways in which we can bring pressure to bear on the regime in Pyongyang to reengage with us on a different footing than the past talks have been held. So yes, we're evaluating all of those options," said Tillerson, the first top American official to openly comment on the likelihood.

Though North Korea was labelled a terrorist sponsor in 1987 following the mid-air bombing of a passenger flight, Pyongyang was taken off the list in 2008 by Washington to re-engage with the country in denuclearisation negotiations.