A US citizen sentenced to 19 years in a Saudi prison for social media posts criticising the kingdom's rulers has been released, his son told AFP on Tuesday.

Saad Ibrahim Almadi, a 72-year-old of Saudi origin, was arrested in 2021 for what his son, Ibrahim, described as "mild" Twitter posts on topics including the war in Yemen and the 2018 killing of journalist Jamal Khashoggi.

Last October, a court handed Almadi a term of 16 years behind bars, which last month was lengthened to 19 years before his surprise release on Tuesday, Ibrahim said.

"Yes, he was freed five hours ago. He's in his Riyadh home," Ibrahim told AFP by phone from the United States.

A travel ban also imposed last year means he cannot leave the country, Ibrahim said.

But he added that he intends to lobby for his father's return to the US so he can receive treatment for medical issues including back problems and diabetes.

"The fight will continue... He needs to come get his medical treatment from the US," Ibrahim said.

Almadi's case has risked further ratcheting up tensions between Riyadh and Washington, long-time partners that have recently been at odds over issues including human rights and oil output cuts approved by the OPEC+ cartel.

The State Department said last year it had "consistently and intensively raised our concerns regarding the case at senior levels of the Saudi government", and that "exercising freedom of expression should never be criminalised".

Saudi officials have not commented on the case.

- Crackdown -
The Gulf kingdom has come under fire for what human rights groups describe as draconian sentences for social media criticism of its policies.

Nourah al-Qahtani, a mother of five in her late 40s, was sentenced last year to 45 years for using Twitter to "challenge" the country's leaders.

Salma al-Shehab, a doctoral candidate at Britain's University of Leeds, was sentenced to 34 years in prison for allegedly aiding dissidents seeking to "disrupt public order" by retweeting their posts.

Ibrahim previously shared with AFP a list of Twitter posts he said had been used in evidence against his father -- information he said had been confirmed by the State Department.

They included posts on taxes as well as controversial demolition work in Mecca, the holiest city in Islam, and the Red Sea city of Jeddah.

One post questions why Saudi Arabia is unable to prevent attacks by Huthi rebels from war-wracked Yemen, where the kingdom heads a military coalition in support of the internationally recognised government.

Another refers to the "sacrifice" of Khashoggi, whose killing by Saudi agents inside the kingdom's Istanbul consulate sparked global outrage.

Saudi officials also found an unflattering caricature of Crown Prince Mohammed bin Salman, the kingdom's de facto ruler, on Almadi's phone, Ibrahim said.

Ibrahim said his father had been instructed by Saudi authorities not to talk about the case -- a claim that could not be independently verified.

"They made him sign a paper to not go public and not talk about his release. I'm the one who is talking about this one, because I want to increase the pressure to bring him back home," he said.

"If it's up to him he's going to stay silent about it. He thinks it's better, but it's not better for him. It's better to talk about it and to fight."