The health of jailed Saudi women's rights activist Loujain al-Hathloul, on hunger strike since last week, is rapidly worsening, and she should be released "immediately", UN experts said Thursday.

Hathloul, whose jailing since 2018 has made her emblematic of the fight for women's rights in Saudi Arabia, began refusing food on October 26, following another week-long hunger strike in August over the conditions of her prolonged detention.

Her deteriorating health was "deeply alarming", the UN Committee on the Elimination of Discrimination against Women (CEDAW) said in a statement.

The committee, made up of 23 independent experts, also voiced serious concern "by recent information concerning the conditions of Ms al-Hathloul's prolonged detention, including reports that she is not allowed regular contact with her family".

Hathloul, 31, was arrested along with around a dozen women activists in May 2018, just weeks before Saudi Arabia lifted a decades-old ban on female drivers.

Some of them have been provisionally released, while others including Hathloul remain in detention amid what campaigners call opaque court trials over charges that include contact with foreign media, diplomats and human rights groups.

The pro-government media branded Hathloul and other jailed activists as "traitors" and her family alleges she faced sexual harassment and torture, including electric shocks and waterboarding, in detention.

Saudi authorities vigorously deny the charges.

Three months prior to her arrest, she had met with CEDAW to share her observations on the state of women's rights in Saudi Arabia.

The rights experts voiced outrage that the national security-related charges against her were in part justified by her engagement with the committee. "Human rights defenders have the right to communicate with the UN, and they should do so free from fear or retribution of any sort," they said.

Saudi Arabia had assured the committee in February this year that Hathloul's trial would take place a month later, but the hearing has been postponed several times since then, according to Thursday's statement.

The experts appealed to Saudi King Salman to help ensure Hathloul's release.