A South Korea politician, who was reportedly briefed on the succession issue by the South's intelligence agency, confirmed Tuesday's newspaper reports.
"I was notified by the South Korean government of such moves and the loyalty pledges," Park Jie-won, a member of the opposition Democratic party, said in a statement.
Park did not name his source, but the South's Yonhap news agency said Park was among a group of lawmakers briefed on Monday night by intelligence officials on the succession plans.
Yonhap quoted another source as saying the request for an oath of loyalty to Kim Jong-un came shortly after North Korea's nuclear test on May 25.
The agency said Kim himself had credited the success of the nuclear blast to Jong-un, although no indication was given as to what role he may have had.
North Korea's state media has hailed the test as a crowning achievement of the "songun" or "military first" policy.
The elder Kim, now aged 67, is thought to have suffered a stroke last year and has been seen in public on only a few occasions since.
He took over from his father, North Korea's founder Kim Il-sung, following his death in 1997 – although Kim Il-sung remains officially North Korea's "eternal president".
Speculation over Kim Jong-il's health and his likely successor has intensified following North Korea's recent nuclear and missile tests that have dramatically raised tensions in the region.
In April, it launched what it says was a satellite into orbit atop a three-stage rocket.
However, the US and its regional allies believe the launch was cover for a test of the North's longest-range missile.
Tensions increased further with last month's test of a nuclear device.
Kim's leadership succession plans – if there are any - are one of the most closely-guarded secrets in North Korea.
If confirmed, the choice of Kim Jong-un could bring potential problems in a society which traditionally places value on seniority.
He is the youngest of three sons Kim Jong-il is known to have fathered.
Analysts have said the North's recent military posturing may be a manifestation of Kim's efforts to solidify his powerbase ahead of formally naming a successor.