The US-based Committee to Protect Journalists (CPJ) says it fears for several journalists and bloggers in Syria who have vanished, with no response from authorities as to their fate.
"We are concerned for the safety of Lina Ibrahim, Wael Abaza and Hussein Ghrer and other journalists who we believe to be in the custody of Syrian authorities," said Mohamed Abdel Dayem, the committee's Middle East and North Africa programme coordinator.
"The government must immediately clarify whether it is holding these journalists, and if so, why."
In a statement on Monday, the CPJ said Lina Ibrahim, a 31-year-old business reporter for the state-owned daily Tishreen, had been missing for six days. A friend, who feared to be named, told CPJ Ibrahim was last seen leaving her home in the Harasta suburb of Damascus last Tuesday.
Freelance journalist Wael Yusef Abaza also disappeared on October 25 in Damascus, the Syrian Center for Media and Freedom of Expression reported. Abaza writes for several newspapers and Arabic news websites.
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Families of both journalists had reported their disappearances to the authorities, who have not responded.
One day earlier, Hussein Ghrer, a prominent blogger, disappeared after leaving his home on October 24, say local and regional press freedom groups. The circumstances of his arrest, his condition and his whereabouts are unknown, according to the Syrian Center for Media and Freedom of Expression.
Shortly before he disappeared, Ghrer wrote on his blog: "Silence doesn't serve us after today. We don't want a country where we get imprisoned for uttering a word. We want a country that embraces and welcomes words."
CPJ believes Ibrahim, Abaza, and Ghrer to be in state custody, although it has not been able to independently confirm their detention.
Security forces detained Omar al-Assad, Rudy Othman and Hanadi Zahlout, all freelance journalists, on August 4 as they attended a Damascus protest. They have not been heard from since.
Another arrest, that of Amer Matar, took place on September 3 in Damascus, CPJ reported. All four journalists remain in detention without charge.
An undetermined number of journalists have been detained by authorities since mass unrest erupted across Syria in March. Many remain in custody.