The Democratic People's Republic of Korea's test-launches of two Musudan missiles on Thursday, although appeared both failed, have added to uncertainty of the situation on the Korean Peninsula.

The DPRK test-fired another Musudan missile from a mobile launcher at about 7:26 p.m. local time (1026 GMT) from the Wonsan area on the country's east coast, Yonhap news agency reported on Thursday, citing South Korea's Joint Chiefs of Staff.

It appeared to have failed as the missile exploded in mid-air several seconds after liftoff.

Earlier in the day, the DPRK test-launched a mobile Musudan missile from the same region, which was also believed to have failed as it crashed in waters off the east coast several seconds after take-off.

The move by Pyongyang was widely watched as it announced on Sunday its successful launch of a ballistic missile from a submarine, which Seoul's military said has partially advanced in technology. The missile traveled about 30 km, indicating an initial stage of development.

The United Nations on Thursday condemned the launch by Pyongyang.

"We would just repeat that these type actions by the DPRK are extremely troubling and we would yet again encourage the DPRK to cease any further provocative actions and return to full compliance of its international obligations," UN spokesman Stephane Dujarric said at a daily news briefing.

The DPRK's first test-firing of the Musudan missile on April 15 also failed as the launched missile also exploded in mid-air.

The Musudan missile, known to be capable of hitting part of the U.S. territory such as Guam and the outer reaches of Alaska, has been deployed by the DPRK since 2007.

The missile is considered especially threatening as it is fired from a mobile launcher, making it hard to detect and track in times of military conflicts. It can also carry a nuclear warhead.

As all of the three test-firings ended in failure, South Korea expects the DPRK to conduct its fifth nuclear test to offset the abortive launches ahead of a historic Workers'Party of Korea (WPK) convention on May 6, the first since 1980.

South Korean President Park Geun-hye convened the National Security Council meeting in the afternoon, the third of its kind in 2015. The first meeting was convened after Pyongyang's fourth nuclear test on Jan. 6, and the second meeting came in the wake of the North's long-range rocket launch on Feb. 7.

Park said in her introductory remarks that signs of the DPRK's fifth nuclear test were detected ahead of the seventh WPK convention.

The National Intelligence Service (NIS), South Korea's spy agency, told lawmakers at a closed-door meeting on Wednesday that the DPRK has actually completed preparations for another nuclear detonation at its main nuclear test site where all of its four nuclear tests were staged.