The soldiers were found guilty of "the crime of high treason in cooperation with the enemy" in a way that threatens the kingdom and its military interests, the ministry said in a statement published by the official Saudi Press Agency.

The statement named the three soldiers -- Mohammed bin Ahmed, Shaher bin Issa and Hamoud bin Ibrahim -- without identifying who they were accused of colluding with.

The announcement comes as Crown Prince Mohammed bin Salman, the 35-year-old heir to the throne, consolidates his grip on power and as a Saudi-led military campaign intensifies in neighbouring Yemen.

Prince Mohammed is already viewed as the country's de facto ruler, controlling all the major levers of government, from defence to the economy.

He holds the title of defence minister, while his younger brother Prince Khalid bin Salman is the deputy.

Over the past three years, the crown prince has mounted a sweeping crackdown on critics and rivals, with the imprisonment of prominent royal family members, business tycoons, clerics and activists.

In March last year, Prince Ahmed bin Abdulaziz al-Saud, a brother of King Salman, and the monarch's nephew Prince Mohammed bin Nayef were detained, multiple sources said, as the crown prince sought to stamp out traces of internal dissent.

Saudi authorities have not publicly commented on their ongoing detention.

Riyadh led a military coalition into Yemen in March 2015 to prop up the internationally recognised government, but it has struggled to oust the Iran-backed Huthi rebels.

It has also faced a surge in missile and drone attacks against the kingdom.

Fighting has also intensified for the key Yemeni region of Marib, with 53 pro-government and Huthi rebel fighters dead in the past 24 hours, loyalist military officials said Saturday.

The Huthis have been trying to seize oil-rich Marib, the government's last significant pocket of territory in the north, since February.