Four US senators have urged President Donald Trump to help free the detained children of a former top Saudi intelligence official exiled in Canada, according to a joint letter released Thursday.

Two adult children and a brother of Saad Aljabri, who is said to hold key state secrets, were detained in Riyadh in March, with a source close to the family telling AFP they were victims of a "Saudi game of thrones." 

Aljabri, credited by Western officials for playing a pivotal role in the kingdom's fight against Al-Qaeda, had earlier attempted to get his children to leave Saudi Arabia but authorities had placed them under a travel ban, the source said.

"We write to express our urgent concern about the abduction in Saudi Arabia of two children of a close US ally and friend, Dr Saad Aljabri," Democratic Senators Patrick Leahy, Tim Kaine and Chris Van Hollen, joined by Republican Senator Marco Rubio, said in a joint letter to Trump.

"The Saudi government is believed to be using the children as leverage to try to force their father's return to the kingdom from Canada, where he currently resides.

"We believe the US has a moral obligation to do what it can to assist in securing his children's freedom. We urge you to raise this issue with senior Saudi officials and press for the immediate release of Dr Aljabri's children," said the letter, posted by Leahy on Twitter.

Saudi authorities have so far not publicly commented on the case.

The source close to the Aljabris said the whereabouts of the children -- Sarah and Omar, who are in their early 20s -- remain unknown and the family's repeated appeals to Saudi rulers have gone unanswered.

- 'Pawns' -
Aljabri is a former top aide to Prince Mohammed bin Nayef, who was edged out as heir to the throne in 2017 by Crown Prince Mohammed bin Salman, the kingdom's de facto ruler.

Prince Nayef and Prince Ahmed -- King Salman's brother –- were also detained by authorities in March. 

They are among a wave of royals detained in recent months as Prince Mohammed, known by his initials MBS, eliminates potential rivals to amass power unseen by previous rulers.

The Aljabri family source said the two children were caught up in the dangerous power plays and being used as "pawns."

Saudi Arabia has been exerting pressure on Canada to extradite  Aljabri even though Ottawa does not have an extradition treaty with the kingdom, The Globe and Mail newspaper reported this week.

Prince Mohammed, who has faced repeated allegations of human rights abuse, is a close ally of Trump.

In their letter, the senators pressed Trump to help Aljabri, describing him as a "highly valued partner" of American intelligence agencies.

"As a top intelligence officer in Saudi Arabia, Dr Aljabri has been credited by former CIA officials for saving thousands of American lives by discovering and preventing terrorist plots," said the letter.

"His development of a modern forensics system in Saudi Arabia reportedly contributed to the significant curtailing of terrorist groups including Al Qaeda."