PARIS (AFP) - The sister of a leading Saudi rights activist jailed since 2018 has urged world leaders to use this weekend's virtual G-20 summit, hosted by Riyadh, to press for her freedom.

Saudi Arabia, which is frequently accused of human rights abuses, is the first Arab nation to host the annual gathering of leaders of the world's biggest economies.

It has been in the crosshairs of campaigners for continuing to imprison several activists, including Loujain al-Hathloul, a key figure in the campaign to allow Saudi women to drive and end the ultra-conservative Sunni kingdom's system of male guardianship of women.

Ms Hathloul, 31, was arrested in May 2018 with about a dozen other women activists just weeks before Saudi Arabia lifted its decades-long ban on female drivers.

Some have been provisionally released, but Ms Hathloul and others remain in detention on what rights groups describe as opaque charges that include contact with foreign media, diplomats and activist groups.

Ms Hathloul has been on a hunger strike since Oct 26. Her sister, Lina al-Hathloul, urged world leaders this week to use the forum "to publicly demand the release of activists languishing in Saudi jails".

"The leaders of the world must be our voice," she told AFP by telephone from Berlin.

The pro-government media has branded Ms Hathloul and others as "traitors", and her family alleges she experienced sexual harassment and torture, including electric shocks and water boarding. Saudi authorities deny the allegations.

According to her sister, Ms Hathloul's family have not had news from her since Oct 26, when she was last visited by her parents and told them she had lost hope and stopped eating.

On Nov 5, UN experts said her deteriorating health was "deeply alarming".

"Today, there is no more physical torture, but they are trying to break her psychologically," said Lina al-Hathloul, citing long periods of solitary confinement and no contact with the outside world.

Their parents are prohibited from leaving Saudi Arabia.

Quiet diplomacy failing

Two years after facing international condemnation over the murder of Saudi journalist Jamal Khashoggi, the kingdom is seeking to use its Group of 20 presidency and the summit to boost its international image and attract foreign investment.

Saudi Arabia's de facto ruler Crown Prince Mohammed bin Salman - MBS for short - has sought to shake off his country's ultra-conservative image in recent years.

He has allowed cinemas, mixed-gender concerts and greater freedoms for women, including the right to drive.

But he is also suspected of involvement in Mr Khashoggi's killing and dismembering at the Saudi consulate in Istanbul - a charge he denies - and is intolerant of opposition.

"The new Saudi Arabia of MBS, we don't really believe in it. The true reformers of this country are behind bars and cannot speak out," Ms Hathloul's sister said.

And she insisted that the "quiet diplomacy" world leaders have been using with Saudi Arabia is not working.

"After three years of arbitrary imprisonment of activists, I think the strategy must change," she said. "We have to take advantage of this summit where Saudi Arabia is going to be in the spotlight for a few days to say things publicly."

The French foreign ministry, asked whether it would raise the matter at the summit, told AFP that "France is constantly mobilising in favour of human rights" and said President Emmanuel Macron had called for Ms Hathloul's release last year.

'A heroine'

"Even though I don't really feel like I'm in danger in Europe, I know the violence of which they are capable, we saw it with Khashoggi," said Lina al-Hathloul, who has lived in Europe for several years.

"I try not to think of it, because that would stop me in my fight," she told AFP.

"I don't know if my sister knows what is going on in the outside world. I just want to let her know that the entire world continues to fight for her freedom, and that we consider her a heroine, not a traitor," Lina al-Hathloul said.