The de-facto number two in the North Korean hierarchy after Kim Jong-Un has been replaced as head of the army's political department, the official KCNA news agency says, signalling a major leadership change.

In a report on May Day celebrations in Pyongyang on Friday, KCNA named Hwang Pyong-So as the director of the Korean People's Army (KPA) General Political Bureau, not Choe Ryong-Hae, who previously held the position.

Hwang's appointment comes just days after KCNA reported his promotion to the rank of vice marshall on April 28 - a rank shared with Choe and four others.

Choe was widely believed to have moved into the role of North Korea's unofficial number two following the execution in December of Kim's uncle and political mentor, Jang Song-Thaek.

Seen as Kim Jong-Un's close aide, Choe assumed the post of vice chairman of the powerful National Defence Commission earlier this month.

Choe had dropped from public sight on several occasions in recent months - once for as long as three weeks - prompting speculation that he might have been purged.

But he reappeared and his replacement as head of the KPA's political department is believed to be down to serious health problems.

Hwang was said in the KCNA report Friday to have addressed a banquet in Pyongyang held for workers at a textile mill, attended by KPA commanding officers.

A report on KCNA earlier in the week said Hwang accompanied Kim on a tour of a newly built workers' hostel at the mill but made no mention of Choe, sparking initial speculation of the leadership shuffle.

Speculation about changes at the top of the Pyongyang regime intensified following Jang Song-Thaek's execution.

In February North Korea promoted a host of key military officials including the chief of its rocket unit, the Strategic Rocket Force Command, which is in charge of the country's mid and long-range missiles program.

Satellite images showing a recent increase in activity at North Korea's main nuclear test site has led to concerns that the country may be preparing to stage a fourth nuclear test.