A US citizen jailed in Saudi Arabia is being punished for "mild" Twitter posts on topics including the war in Yemen and the 2018 killing of journalist Jamal Khashoggi, his son told AFP on Wednesday.

Saad Ibrahim Almadi, a 72-year-old of Saudi origin, was this month sentenced to 16 years in prison, the latest in a spate of what human rights groups describe as draconian sentences for social media criticism of the government.

The case risks further ratcheting up tensions between Riyadh and Washington, longtime partners currently at odds over oil output cuts approved by the OPEC+ cartel, which the White House says amount to "aligning with Russia" in the Ukraine war.

Almadi was detained on arrival in Saudi Arabia in November last year for what was meant to be a two-week trip, said his son Ibrahim, who went public with the case this week, criticising US officials for failing to do more to secure his release.

The State Department said on Tuesday 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".

On Wednesday, Ibrahim 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 include posts on taxes as well as controversial demolition work in the Red Sea city of Jeddah.

One post questions why Saudi Arabia is unable to prevent attacks by Huthi rebels in 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 in 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.

Almadi was charged in part with supporting and funding terrorism and trying to destabilise the kingdom, Ibrahim said.

Riyadh has not commented on the case.