Jailed Saudi activist Loujain al-Hathloul appeared before a terrorism court on Thursday, her family said, as she faces the prospect of a lengthy prison sentence on what UN experts called "spurious" charges.

Hathloul's hearing comes two days after a prominent American-Saudi doctor, Walid Fitaihi, was sentenced to six years in prison, highlighting what campaigners call growing state repression, despite international pressure over the kingdom's human rights record.

Hathloul, 31, was arrested in May 2018 with about a dozen other women activists just weeks before the historic lifting of a decades-long ban on female drivers, a reform they had long campaigned for.

After being tried in Riyadh's criminal court, her trial was transferred last month to the Specialised Criminal Court (SCC), or the anti-terrorism court, which campaigners say is notorious for issuing long jail terms and is used to silence critical voices under the cover of fighting terrorism.

"Loujain's spirits are high, but her body is still weak," her sister Alia al-Hathloul said after a hearing in the SCC.

Hathloul's siblings are based outside the kingdom, but some of her other family members are allowed to attend court hearings, which are off-limits to journalists and diplomats.

"We are extremely alarmed to hear that Al-Hathloul, who has been in detention for more than two years on spurious charges, is now being tried by a specialised terrorism court," said Elizabeth Broderick, chair of the United Nations working group on discrimination against women and girls.

"We call once again on Saudi Arabia to immediately release Al-Hathloul, a woman human rights defender who has greatly contributed to advancing women's rights," she added in a statement.

Foreign Minister Prince Faisal bin Farhan told AFP last week that Hathloul is accused of contacting "unfriendly" states and providing classified information, but her family said no evidence to support the allegations had been put forward.

While some detained women activists have been provisionally released, Hathloul and others remain imprisoned on what rights groups describe as opaque charges.

The pro-government Saudi media has branded them as "traitors" and Hathloul's family alleges she experienced sexual harassment and torture in detention. Saudi authorities deny the charges. 

Saudi Arabia, an absolute monarchy, faces growing international criticism for its human rights record.

But the kingdom appears to be doubling down on dissent, even as US President-elect Joe Biden's incoming administration could intensify scrutiny of its human rights failings.

On Tuesday, the Saudi terrorism court sentenced Fitaihi, 56, to six years in prison, on charges including getting US citizenship without permission from authorities and sympathising with an unnamed terrorist organisation, a source close to his family told AFP.

Saudi authorities have not publicly commented on his case, and it remains unclear why those charges constitute a crime.

"Saudi authorities' railroading of Fitaihi under broad charges shows that the government has no intention of loosening its clampdown on peaceful critics," said Adam Coogle, deputy Middle East director at Human Rights Watch.