President Donald Trump is due to meet with North Korean leader Kim Jong Un on June 12 in Singapore with Pyongyang's nuclear arsenal high on the agenda and reports said that the reshuffle could be aimed at taming the military.

Late last month the North's state media revealed that Kim Su Gil had been appointed as director of the military's powerful General Political Bureau (GPB), replacing Kim Jong Gak.

According to Yonhap news agency, which cited intelligence sources, the chief of the general staff Ri Myong Su has also been replaced by his deputy, Ri Yong Gil.

And defence minister Pak Yong Sik has been succeeded by No Kwang Chol, previously first vice minister, it added.

The wholesale reshuffle would be unusual if confirmed, Seoul's unification ministry said.

"We will monitor related developments," ministry spokesman Baik Tae-hyun told reporters.

Pyongyang's military, known as the Korean People's Army, is immensely influential in the North and a centre of power in its own right, symbolised by the way Kim is habitually flanked by generals on one side and civilians on the other when attending major ceremonial events.

According to researchers at NK Leadership Watch, the change at the top of the GPB "represents a continuation of tightening Party control over the KPA".

The political bureau could be in a position to resist policy decisions by the leadership or try to profit from future South Korean economic aid, it said.

But new GPB director Kim Su Gil was a "highly trusted" lieutenant of leader Kim Jong Un, it added, who appointed him to the Pyongyang party committee -- once a powerbase for his uncle Jang Song Thaek -- after having the older man executed for treason in 2013.

Reports said the wider changes could be aimed at preventing objections in the North's senior military ranks to any sudden changes in the country's nuclear policy.

No Kwang Chol, the new defence minister, was known as a "moderate", Yonhap cited the intelligence source as saying.

"The North appears to have brought in new figures... as the previous officials lacked flexibility in thinking," the source said.