The United Nations human rights office has called on Saudi Arabia on to release all peaceful activists, including women held for campaigning against a ban on driving even as it was being lifted.

At least 15 government critics were arrested since mid-May, some of whose whereabouts are unknown amid a serious lack of transparency in the processing of their cases, the rights office said on Tuesday.

They included prominent women's rights advocate Hatoon al-Fassi, arrested in June as she was planning to take journalists in her car to celebrate the much-hyped end of the world's last ban on female drivers, long seen as an emblem of repression in the deeply conservative Muslim country.

"We urge the Government of Saudi Arabia to unconditionally release all human rights defenders and activists who have been detained for their peaceful human rights work, including their decades-long campaigns for the lifting of the driving ban for women," UN human rights spokeswoman Ravina Shamdasani said.

Crown Prince Mohammed bin Salman, the heir to the throne set to become the first Saudi king from a new generation after a succession of six brothers dating back to 1953, has initiated broad reforms to diversify the economy from oil and update deeply conservative social norms.

But critics say the reforms have not extended into politics in an absolute monarchy where all public opposition to the authorities is still banned.

Genuine reforms appear to be taking place in the kingdom, "but this has not extended to the civil and political rights sphere", Shamsadani told a Geneva news briefing

"Dissent, criticism of the government is still not accepted in the country. That can explain why many of these human rights defenders and activists have been jailed. All of them have criticised government policies in one way or another," she said.