Human rights group Amnesty International has urged the Saudi Arabian government to make public the location of two missing Qatari citizens believed to have been detained last month.
The Qataris disappeared while visiting their family members in the eastern part of Saudi Arabia in mid-August and have not been in touch with their family since.
According to a letter published on Tuesday, Amnesty said it had reasons to believe Ali Nasser Ali Jarallah, 70, and his 17-year-old son Abdulhadi Ali Nasser Ali Jarallah have been detained.
In August, Qatar's state-linked National Human Rights Committee (NHRC) said the two were "forcibly disappeared".
In its letter, Amnesty asked the Saudi King Salman to give out more information about the two Qataris, who also hold the Saudi nationality and entered the country on valid family visas to visit Ali Jarallah's brother in Dammam in the Eastern Province.
"The two men were in contact with their family in Qatar until August 18, when they were nearing the city of al-Hofuf in the Eastern Province," the letter said.
"Since then all communication was lost and they disappeared."
The Amnesty letter said the mobile phones the two were using remained connected for another three days, but neither responded to messages.
According to the letter, the elderly Jarallah is a diabetic who suffers from high blood pressure and heart and kidney problems.
"He is on regular medication and must attend his regular doctor's appointments in Doha," Amnesty said.
Amnesty said the "whereabouts" and the reason for the detention of the two Qataris should be revealed by the Saudi authorities "in accordance with the principles of due process recognised by international law".
"In the meantime, Ali Nasser Ali Jarallah must be immediately granted all medical access his situation requires," said the letter.
The relationship between Saudi Arabia and Qatar has been strained for more than two years.
In June 2017, Saudi Arabia, along with the United Arab Emirates, Bahrain and Egypt, accused Doha of violating a 2014 agreement with members of the Gulf Cooperation Council by meddling in regional affairs and "supporting terrorism".
Qatar has strenuously denied the allegations and views the blockade as an attack on its sovereignty.
Over the last two years, Qataris wanting to travel to Saudi Arabia have had troubles obtaining a visa, even for the annual Hajj pilgrimage.
