Reported detentions mark latest crackdown by Crown Prince Mohammed bin Salman, the kingdom's de facto ruler.
Saudi Arabia has detained two senior members of the royal family - Prince Ahmed bin Abdulaziz, the younger brother of King Salman, and Mohammed bin Nayef, the king's nephew - according to reports citing sources with knowledge of the matter.
The Wall Street Journal reported the detentions of the two royals on Friday and said they related to an alleged coup attempt. Bloomberg also reported the detentions, quoting a source as saying that the pair were accused of "treason".
Mohammed bin Nayef's younger brother, Prince Nawaf bin Nayef, had also been detained, according to the New York Times.
There was no immediate comment by Saudi authorities.
The detentions mark the latest crackdown by Crown Prince Mohammed bin Salman, King Salman's son and the de facto ruler of the kingdom.
Prince Mohammed, also known as MBS, has moved to consolidate power since replacing his cousin, Mohammed bin Nayef, as heir to the throne in 2017. Later that year, he arrested dozens of royals and business people, in what was billed as a move against corruption that was draining state coffers.
But the crown prince has fuelled resentment among some prominent branches of the ruling family by tightening his grip on power and some question his ability to lead following the 2018 murder of prominent journalist Jamal Khashoggi by Saudi agents and a major attack on Saudi oil infrastructure last year, sources have told Reuters news agency.
The sources said royals seeking to change the line of succession view Prince Ahmed, King Salman's only surviving full brother, as a possible choice who would have the support of family members, the security apparatus and some Western powers.
In late 2018, a video emerged of Prince Ahmed facing protesters outside his London residence and in which he seemed to criticise King Salman and his crown prince for the war in Yemen.
"Don't blame the entire family ... Those responsible are the king and his crown prince" he said. "In Yemen and elsewhere, our hope is that the war ends today before tomorrow."
Though Prince Ahmed quickly retracted his comments, insisting that his words were taken out of context, messages of support and pledges of allegiance began pouring in.
The 78-year-old also issued a statement to deny speculation that he was interested in the role of monarch.
Prince Ahmed has largely kept a low profile since returning to Riyadh in October 2018 after two and a half months abroad.
He was one of only three people on the Allegiance Council, made up of the ruling Al Saud family's senior members, who opposed Mohammed bin Salman becoming crown prince in June 2017, sources earlier said.
Saudi insiders and Western diplomats say the family is unlikely to oppose MBS while the 84-year-old king remains alive, recognising that he is unlikely to turn against his favourite son. The monarch has delegated most responsibilities of rule to his son but still presides over weekly cabinet meetings and receives foreign dignitaries.
Al Jazeera's Jamal Elshayyal, commenting from Doha, said the detentions were of "huge" significance.
"We are talking about two of the most senior members of the Saudi royal family," he said.
"What's prompted it is very difficult to ascertain, needless to say, because Saudis have a closed culture in terms of transparency and no media freedom.
"But these are two figures who have been under house arrest. They haven't been able to move freely for a very long time. The idea that they were trying to hatch some sort of coup is very far-fetched and difficult to see when considering the restraints they were under."
'Sign of nervousness'
The latest detentions come at a time of heightened tension with regional rival Iran and as Prince Mohammed implements ambitious social and economic reforms, including an initial public offering by oil giant Saudi Aramco on the domestic bourse last December.
Saudi Arabia is also the current chair for the Group of 20 (G20) major economies.
MBS has been lauded at home for easing social restrictions in the kingdom and opening up the economy.
But he has come under international criticism over a devastating war in Yemen, the murder of Khashoggi in the kingdom's Istanbul consulate and the detention of women's rights activists seen as part of a crackdown on dissent.
"Prince Mohammed is emboldened - he has already ousted any threats to his rise, and jailed or murdered critics of his regime without any repercussion," Becca Wasser, a policy analyst at the US-based RAND Corporation, said of the latest crackdown.
"This is a further step to shore up his power and a message to anyone - including royals - not to cross him."
Rami Khouri, a journalism professor at the American University of Beirut, echoed Wasser's sentiment, saying the idea of a coup being fomented was unlikely in light of the "immense, direct and brutal control" that the crown prince has over all of the kingdom's security agencies.
"It is a sign of the nervousness of the crown prince and the people around him who rule Saudi Arabia because they probably expect that the king will either abdicate or pass away soon. They expect there might be some kind of challenge to the succession," Khouri said.
"The critical thing about this, I think, is that it is the final affirmative confirmation, the seal on Mohammed bin Salman taking over the mantle of the Arab autocrats that used to be held by people like Saddam Hussein, Muammar Gaddafi and Hafez al-Assad."