TRIPOLI, Libya — A website that supports the Syrian government publicized a video clip Monday that showed Austin Tice, a US freelance journalist, to be alive but held hostage by what appeared to be Islamist militants. It was the first glimpse of Tice since Aug. 13.

Questions about the origin of the undated clip and the anomalies in it raised doubts about its authenticity, but colleagues and relatives confirmed that a masked figure with long hair and a scruffy beard appeared to be Tice, 31, a former Marine whose work has been published in McClatchy Co. newspapers, The Washington Post, and other news outlets.

The 47-second video, with the headline ‘‘Austin Tice Still Alive,’’ shows frightening scenes of masked gunmen jerking Tice along a trail through low hills. One captor holds what looks to be a rocket-propelled grenade launcher.

Tice kneels and the men force him to repeat in clumsy Arabic the prayer that Muslims traditionally recite before dying. Tice then says in English, “Oh, Jesus. Oh, Jesus,’’ sounding breathless and frightened. Then he lowers his head, and the video ends with him, unhurt, resting his head on the arm of a captor.

Several analysts said that the video appeared to be staged and that it lacked the customary form and polish of jihadist videos. The men hid their faces, and no group was identified claiming responsibility for Tice’s capture or the video, which was posted on YouTube by an unknown user instead of over a jihadist website, as militant groups prefer.

In the video, the call-and-response of ‘‘God is great’’ seems unpracticed and out of sync. The captors are dressed in freshly pressed Afghan dress never seen before among Syrian rebels. And it was unusual for Islamist militants to force Tice, a non-Muslim, to recite a Muslim prayer for a video.

The State Department has said it believes that Tice is in the custody of the Syrian government, which has now acknowledged holding him. Links to the video appeared Monday on a Facebook page and Twitter account supporting Syria’s president, Bashar Assad, whose government contends it is fighting foreign-financed Islamist militants in the country’s increasingly brutal civil war. Online postings with the video cited it as evidence of that. The accompanying Twitter post said, ‘‘Important, please publish and share our clip on the truth about the disappearance of the American journalist Austin Tice.’’

The Facebook posting declared that ‘‘Austin Tice is with the Nusra Front gangs and Al Qaeda in Syria,’’ a well-known group of Islamist Syrian opposition fighters. But the group releases its own videos through its own channels, and if this clip has been produced by a militant opposition group, it was unclear why it was being disseminated on pro-Assad websites.

Tice entered Syria in May without official authorization, crossing over the Turkish border into rebel-controlled territory. Since July, he had reported from the contested Damascus suburbs. He last exchanged e-mail with colleagues Aug. 13.

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