A regional official of the Committee to Protect Journalists said there was "strong evidence" to suggest the journalist, Dorothy Parvaz, had been detained on arrival at Damascus airport on a flight from Qatar. She has U.S., Iranian and Canadian citizenship, and formerly was a reporter and columnist for the Seattle Post-Intelligencer.
"We are deeply concerned for Dorothy's safety, security, and well-being," Al-Jazeera said in a statement. "We are requesting full cooperation from the Syrian authorities to determine what happened at the airport, what her current location is, and the status of her health."
Parvaz joined Al-Jazeera in 2010 and recently reported on the Japanese earthquake and tsunami. The station said she graduated from the University of British Columbia, obtained a masters from Arizona University, and held journalism fellowships at both Harvard and Cambridge universities.
"Obviously we are worried for the safety of Dorothy, specifically, as we are for numerous other journalists who are in government custody right now," Dayem said.
Some Syrian journalists have been in custody for weeks as part of an effort by the Syrian government limit media coverage of the unrest, he said.
Once-unimaginable protests are posing the most serious challenge to four decades of rule by the Assad family in one of the most repressive countries in the Middle East.
The Post-Intelligencer quoted Parvaz's fiance, Todd Barker, as saying he spoke to her by telephone the night before she left for Syria, and she had expressed no concerns about traveling there.
Barker and Parvaz's family issued a statement appealing for information about her; it described her as a "global citizen" and a "determined journalist."