A Saudi prince detained in a guarded villa in Riyadh without any formal charges has been moved to an "undisclosed location", a European lawmaker said in a letter seen by AFP Wednesday.

The move marks an escalation in the nearly three-year detention of Prince Salman bin Abdulaziz, highlighting the kingdom's defiance against international pressure for his release even as US President-elect Joe Biden's incoming administration could intensify scrutiny of its human rights record.

In January 2018, Prince Salman, now 37, was swept up along with his father in a wide-ranging crackdown, leaving his supporters asking why the minor royal who posed no apparent challenge to powerful Crown Prince Mohammed bin Salman was targeted.

Prince Salman, educated at Paris' Sorbonne University, and his father were held in solitary confinement for around a year in the high-security Al-Hai'r prison near Riyadh and later in a guarded villa in the capital, two sources close to him said.

The prince was moved to a secret detention site in March but was mysteriously returned two months later to the villa, which is under high security surveillance, the sources said, after a $2 million US lobbying effort and petitions from European lawmakers calling for his release.

Last Saturday, the prince and his father were removed from the villa and "taken away to an undisclosed location", lawmaker Marc Tarabella said in a letter to the Saudi ambassador to the European Union.

"It is clear that their current deprivation of liberty is arbitrary, and amounts to a violation of Saudi domestic and international obligations," said the letter dated Tuesday.

"I urge you to ask the Saudi government to provide immediately the whereabouts" of Prince Salman and his father Abdulaziz bin Salman, it added.

The letter from Tarabella, the vice chairman of the European Parliament's delegation for relations with the Arab peninsula, was also sent to the Saudi embassies in France and Belgium, a parliament source told AFP.

Saudi authorities have so far not commented publicly on the case, and did not respond to AFP's request for comment on the alleged transfer.

They have disappeared

One of the sources close to the detained prince and his father also said that they had been moved to a secret location.

"No one knows where they moved them," this source told AFP. The two men were allowed regular phone calls to their family, but there has been no communication since Saturday. "They have disappeared," the source added.

Prince Salman is one of many royal family members incarcerated since the meteoric rise of Prince Mohammed, the kingdom's de facto ruler who has mounted a sweeping crackdown on dissenters, critics and political rivals.

In August, two rights groups -- the Geneva-based MENA Rights Group and the London-based ALQST -- lodged a complaint to the UN over their detention, according to a document seen by AFP.

The prince and his father have never once been interrogated since their detention, adding to the arbitrariness of their detention, the rights groups and sources close to them told AFP.

Observers say what may have irked the royal court was Prince Salman's meeting with Democratic Congressman Adam Schiff just before the US elections in 2016.

Schiff is a critic of US President Donald Trump, a staunch backer of Prince Mohammed. 

During his election campaign, Biden vowed to turn the kingdom into a "pariah".

But Saudi Arabia, which hosted the G20 summit last month with barely any public mention of human rights from the leaders of the world's wealthiest nations, appears in no mood to bow to the international pressure.