Move comes following regional downgrade in ties in 2016, after Iranian protesters attacked Saudi diplomatic missions in Iran following Riyadh's execution of cleric Nimr al-Nimr

Little is known about detainee, who has not been identified by Islamic Republic, nor has Sunni kingdom officially announced an arrest

Ali al-Nimr's uncle Nimr al-Nimr was put to death in 2016, in mass execution that sparked downturn in ties between Tehran and Riyadh