South Africa's Zulu king, head of the country's most influential traditional monarchy, said on Monday he was "fit and sound," amid concerns over his health after the suspected poising of a close adviser.

"I'm very well, I'm very fit and sound," Misuzulu Zulu told AFP on the phone speaking from the neighbouring country of Eswatini, where he is undergoing medical checks.

The 48-year-old king ascended the throne last year after the death of his father, Goodwill Zwelithini, amid a bitter feud over the royal succession.

Overnight on Saturday, the influential Zulu prime minister, Prince Mangosuthu Buthelezi, said in a press release that the monarch had been hospitalised in Eswatini after falling ill.

But asked by AFP if he was ever in a hospital the king said "never".

"It was a procedural routine check-up that I do every three months, sometimes every six months," he said in a terse interview.

The royal spokesman, Prince Africa Zulu, told AFP Monday that the Zulu king had spent the day working, including meeting a Chinese delegation, but gave no further details.

Although the title of king of the Zulu nation does not bestow executive power, the monarch wields great moral influence over more than 11 million Zulus, who make up nearly a fifth of South Africa's population of 60 million people.

King Zwelithini, who died after more than 50 years in charge, left six wives and at least 28 children.

Misuzulu is the first son of Zwelithini's third wife, who he designated as regent in his will.

The queen however died suddenly a month after Zwelithini, leaving a will naming Misuzulu as the next king -- a development that did not go down well with other family members.