North Korea's top three military officials have been replaced ahead of an historic summit between leader Kim Jong Un and US President Donald Trump, according to multiple reports.

All three appear to have been replaced by younger Kim loyalists, part of an ongoing transformation of the country's political and military establishment since the young leader took power in 2011.

South Korea's Yonhap news agency, citing an unnamed intelligence official, reported that defense chief Pak Yong Sik had been replaced by No Kwang Chol, while Ri Myong Su, chief of the Korean People's Army (KPA) general staff, had been replaced by his deputy, Ri Yong Gil.

Army general Kim Su Gil's replacement of Kim Jong Gak as director of the KPA's general political bureau was previously referenced in North Korean state media and confirmed Monday by South Korea's Unification Ministry, which deals with North Korean affairs.

"All these (promoted) guys are top Kim Jong Un guys," said Michael Madden, author of the highly respectedNorth Korea Leadership Watch blog. "All three of them have held very sensitive and high-level positions under Kim Jong Un, they're very loyal (to him), and all have experience interacting with foreign delegations."

The three men replacedPak,RiandKim, are 68, 81 and 77 years old respectively.

Shoring up power

Madden said the reshuffling at the top of the North Korean military was likely done for a number of reasons, among them prepare for the Kim-Trump talks and future South Korean negotiations and exchanges.

In particular, the military'sGeneral Political Bureau(GPB) is responsible for auditing and overseeing the financial operations of the KPA, which controls a large number of trading corporations and other businesses which could be highly involved in any future inter-Korean trade or infrastructure projects.

Political commissars under the bureau are stationed throughout the KPA and can influence the army's activity at all levels.

"(Kim) is not going to want these military commissars helping themselves to any of this assistance coming to the North," Madden said. "That was a problem during the sunshine period, a lot of misappropriation and malfeasance."

He added that the size and breadth of the GPB responsibilities are such that, more than any other North Korean organization, it presents the most realisticpotential threat to Kim's own power.

"From about June 2017 to earlier this year, the KPA general political bureau was under investigation by the (ruling Worker's Party), the first time in 20 years that the GPB was under investigation," Madden said.

By bringing the GPB firmly under the Party's and his own control, Kim is likely looking to avoid a repeat of the extreme action he had to take against his uncle, Jang Song Thaek, early into his rule.

Jang, previously one of the most powerful men in North Korea, waspurged and executed in 2013after he reportedly built up an alternate power base to his nephew.