North Korean leader Kim Jong-un appears to have replaced his chief official in charge of domestic security affairs, a Seoul intelligence source said Tuesday, in what could be part of his moves to cement his power.

Ri Myong-su, the North's minister of people's security, has not been seen in public for months, raising speculation that he might have been purged.

"To my knowledge, North Korea recently appointed Choi Bu-il, deputy chief of staff at the North's military, to the minister of people's security," the source said, speaking on the condition of anonymity.

The source declined to give further details, including exactly when Cho was named, but the apparent replacement is believed to be linked to a "part of loyalty test by Kim Jong-un."

Kim, who took over North Korea in late 2011 after the death of his father Kim Jong-il, sacked the North's army chief Ri Yong-ho in July last year, which some analysts said was a sign of the new leader tightening his control over the military.

Ri is regarded as a close confidant of the late Kim.

North Korea set off its third nuclear device on Feb. 12, drawing a chorus of international condemnation. Seoul and Washington are pushing for more sanctions against Pyongyang, which has threatened to "take the second and third stronger steps in succession" to retaliate against such sanctions.

The North's latest atomic test, which came weeks after an apparent successful launch of a long-range missile, raised fears that Pyongyang might have taken a step closer to flying nuclear warheads atop inter-continental ballistic missiles. (Yonhap News)