RSF "is very concerned about the arbitrary detention of Omani journalist and writer Sulaiman al-Moamari, who has been held in solitary confinement ever since his arrest by intelligence officials on 28 April without any official reason being given", a statement said.
It quoted local media as saying the 42-year-old heads the cultural department at Omani state radio and is popular due to "his writings and pro-democracy views".
On the evening of April 28, Moamari was summoned for questioning by the intelligence service.
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"The authorities must give their reasons for arresting Sulaiman al-Moamari and must say where they are holding him," said RSF's Alexandra El-Khazen.
"We urge the Omani authorities to either bring charges against him, so that he has the right to a fair trial, or free him at once," she added.
According to RSF, several Omani and other Arab journalists and writers "have voiced outrage at his detention, which is seen as tantamount to enforced disappearance."
It said that a petition demanding his release has been posted online.
"Censorship and obstruction of freedom of information are common in the Sultanate of Oman, which is ranked 125th out of 180 countries in RSF's 2016 World Press Freedom Index," the watchdog added.