Princesses Sahar, 42, and Jawaher, 38, said that they are being kept against their will in a guarded villa in the royal compound in Jeddah, the Daily Mail reported on Sunday.

Their claims shed light into the usually secret world of royal family of a country where women are effectively treated as second-class citizens.

Saudi Arabia is the only country in the world that prohibits women from driving. It scored 130th out of 134 countries analyzed by the World Economic Forum in a 2009 report on gender parity.

But the restrictions allegedly placed on Sahar and Jawaher go well beyond what is allowed under Saudi law.

In emails and phone calls to a Sunday newspaper, Sahar and Jawaher claimed that their sisters Hala, 39, and Maha, 41, are also being held, incommunicado, in separate villas in the Jeddah compound.

Their mother Alanoud al-Fayez, who is divorced from Saudi King Abdullah, has reportedly written to the UN's human rights agency to intervene on their behalf.

She told the Office of the High Commissioner for Human Rights (OHCHR) that her daughters are 'imprisoned, held against their will, cut off from the world', according to a report in the Sunday Times.

Sahar and Jawaher told the paper in an email that they are kept alone in a house most of which they have closed off as they have been left to fend for themselves with nobody to help them with the housework.

'We slowly watch each other fading away into nothingness,' they said, adding that their sister Hala had told them 'that her mind is slipping away ... that the life is being sucked out of her.'

The king, who has 38 children by a number of wives, has placed his four daughters with al-Fayez under the control of three of their half-brothers, according to Sahar.

Under Saudi law, girls and women are forbidden from travelling, conducting official business, or undergoing certain medical procedures without the express permission of their male guardians.

The OHCHR said that it would pass on al-Fayez's letter to Rashida Manjoo, who is the UN special rapporteur on violence against women.

NTJ/HH