"Recently, (the DPRK's) preparations for the fifth nuclear test are being detected," Park told a meeting with senior presidential secretaries.

Park said the DPRK could conduct any abrupt provocations to escape isolation and solidify domestic loyalty following the international community's additional sanctions on Pyongyang.

The UN Security Council approved tougher-than-ever sanctions on the DPRK in early March over Pyongyang's fourth nuclear test and long-range rocket launch.

The DPRK detonated what it claimed was its first hydrogen bomb on Jan 6, followed by the launch of a long-range rocket, which was condemned as a disguised test of ballistic missile technology, on Feb 7.

From early March, Pyongyang raised tensions on the Korean peninsula by firing short-range and medium-range missiles and artillery shells in a show of force against annual US-South Korea war games, which the DPRK considers as a dress rehearsal for northward invasion.

The Key Resolve command post exercise, which kicked off on March 7, ended about 10 days later, but the Foal Eagle field training exercise is scheduled to last later this month.

Park instructed the military to maintain strong defense readiness to sternly retaliate against any DPRK provocations at any time, calling for support from opposition parties in issues relevant to national security.

South Korea's defense ministry also warned of another DPRK nuclear test, saying there is possibility for an underground nuclear test.

Yonhap news agency cited South Korean and the US intelligence authorities saying that the moves of vehicles and personnel recently increased two to three times more than last month at the Punggye-ri nuclear test site in the DPRK's northeast region where the country carried out all of its four nuclear bomb tests.

Concerns mounted about another nuclear detonation since top DPRK leader Kim Jong Un gave orders on March 15 to test a nuclear warhead and ballistic rockets capable of carrying the warhead.

South Korea's defense ministry spokesman Moon Sang-Kyun told a press briefing that the military has focused on Kim's order, saying the DPRK has been in a state of conducting its fifth nuclear test at any time.

Seoul had predicted two scenarios in the DPRK's fifth nuclear test. One is to detonate a nuclear warhead at its underground test site, and the other is to test the detonator of a warhead, from which nuclear materials are eliminated, according to South Korean Defense Minister Han Min-Koo's recent remarks.

If the DPRK conducts its fifth nuclear test in the near future, it would be unusually short in a time gap between tests compared with previous cases. Pyongyang tested nuclear bombs in October 2006, May 2009, February 2013 and January 2016 respectively.

Some of South Korean experts predicted the DPRK's fifth nuclear test to happen possibly before the historic ruling Workers' Party of Korea (WPK) congress scheduled in early May.

Meanwhile, the DPRK launched a Musudan ballistic missile for the first time on Friday. The missile, which was fired from a mobile launcher, was known to have failed as it exploded in mid-air several seconds after liftoff.

The Musudan missile is capable of carrying a nuclear warhead and reaching parts of the U.S. territory such as Guam and the outer reaches of Alaska.