Family and colleagues of a freelance reporter from Texas traveling in Syria say they have not heard from him more than a week.
Austin Tice, 31, a student at Georgetown University Law School and a former Marine, had traveled to Syria for several months to report on the 17-month uprising, filing for a number media outlets including the Washington Post and McClatchy newspaper group.
"It's not uncommon for various journalists moving in and about Syria to be out of communication," Tice's father told The Associated Press Thursday from Houston. "We're very hopeful that that is what is happening."
The Washington Post said it was "focusing intensively on trying to ascertain his whereabouts and ensure his safe return." It praised Tice for his "important, on the ground reporting."
McClatchy said it had enlisted the help of the State Department, which said it is working with the Czech embassy since its own has closed.
"Journalists like Austin take risks every day to deliver the news to the rest of the world," said McClatchy's vice president of news, Anders Gyllenhaal. "Austin's reporting on the events in Syria has been particularly powerful and revealing - a reminder of why this work is so vital."
Journalist visas are very difficult to come by for Syria, and as the conflict drags on, more and more journalists have taken to crossing the borders illegally to cover the uprising. The Committee to Protect Journalists calls Syria the most dangerous place in the world for reporters to work, counting 16 who have been killed since November.
On Monday, Japanese reporter Mika Yamamoto was shot and killed during the heavy fighting raging in Syria's commercial center of Aleppo. Two other journalists with her at the time - a Palestinian correspondent and his Turkish photographer - are now missing. The wife of the Palestinian, Bashar Fahmi, who works for the American-funded Al Hurra network, says she was told by witnesses that they are alive and being held by forces loyal to President Bashar al-Assad.
"We know he was with the Japanese journalist who was killed, because we saw him in the footage that was taken. Everyone saw it," Arzu Kadumi told the AP. "The only thing we know is that they are alive. Otherwise we don't know anything. We are just waiting."
Tice left the Marine Corps in January after serving as an infantry officer. The Post says he crossed into Syria from Turkey in May and made his way down to Damascus by July. He was spending time with rebel fighters and planned to leave Syria in mid-August.
"It's nice and all, but please quit telling me to be safe," he wrote in a July 25 post on his Facebook page. In it, he accused the U.S. of becoming "a fat, weak, complacent, coddled, unambitious and cowardly nation."
"I don't have a death wish - I have a life wish," he wrote. "So I'm living, in a place, at a time and with a people where life means more than anywhere I've ever been - because every single day people here lay down their own for the sake of others. Coming here to Syria is the greatest thing I've ever done, and it's the greatest feeling of my life."