A traditional Ugandan ruler named as the world's youngest monarch has died aged 34, his prime minster announced.

Oyo Nyimba Kabamba Iguru Rukidi IV became king of Tooro in western Uganda at just three years old in 1995 after the death of his father.

The Tooro kingdom did not disclose the cause of King Oyo's death, but a Ugandan government minister was quoted in local media saying that he had been critically ill and was seeking treatment in the US.

Though Uganda is governed by an elected president and parliament, it has several traditional kingdoms which continue to have significant cultural influence despite not having political power.

Oyo, who Guinness World Records listed as the world's youngest reigning monarch, ruled over more than two million people. He was the 13th monarch of a kingdom that was founded in the early 19th Century.

Announcing his death, Tooro's Prime Minister Calvin Armstrong Rwomiire Akiiki called his passing on Thursday an "immeasurable loss" to the royal family and the people of Tooro.

"I call upon all the people of Tooro to remain united, calm and prayerful during this profoundly difficult time," Akiiki said on Friday. "May His Majesty the King rest in eternal peace."

The Tooro Kingdom follows a hereditary system in selecting its monarch.

Oyo was not officially married, but Akiiki said "the continuity of the Crown is assured".

"His Majesty leaves behind a Prince as his heir, whose identity will be formally revealed at the appropriate time, in accordance with the traditions and customs of Tooro Kingdom," he explained.

When he became king in 1995 a regency, which included the queen mother, was established until Oyo turned 18 when there was an official coronation.

The young king was well respected by his people and according to a report in the Daily Monitor newspaper he promoted causes such as youth empowerment, education and climate conservation, as well as HIV/Aids awareness.

The country's largest and most influential traditional institution, the Buganda Kingdom, said it mourned with the people of Tooro following the death of their monarch, labelling him a valuable figure to Uganda.

Similarly, the Rwenzururu Kingdom also extended its condolences to "all those mourning this great loss", according to local media reports.

Others have also paid their respects to the late king.

The Federation of Uganda Football Associations, honoured King Oyo as a "true friend of the beautiful game", and announced a moment of silence before the kick-off of all matches until 4 September in memory of his "legacy, love, passion and contribution" to the sport.

Government minister Margaret Muhanga described the mood in the kingdom as "sombre... you can't see anyone smiling".

The king was "at the prime of his life", she said in a video shared on social media, "as young as he was, he had an impact on his kingdom".

Muhanga said they would be planting a million trees in his honour during the mourning period, and wants to ask the neighbouring districts in Tooro to do the same.

Uganadan musician Juliana Kanyomozi called it a "very dark day for Tooro", in an Instagram post. "Gone too soon. What a loss!!!!"

Meanwhile, pop star-turned-politician Bobi Wine, called for King Oyo's "soul to rest in eternal peace".