The government is sensitive to any news of possible Japanese captures abroad after Islamic State militants last year claimed to have beheaded war correspondent Kenji Goto, a week after the group also said it killed Haruna Yukawa, a self-styled contractor.
The video of Jumpei Yasuda, widely shown on Japanese TV news programmes today, was also available on Facebook.
It was posted online by a Syrian man who lives in Turkey who said Yasuda was taken hostage by the al-Qaida-linked Nusra Front rebel group in Syria, public broadcaster NHK and the Asahi Shimbun daily reported, citing telephone conversations with the man.
"We are aware of the video and are analysing it," Japanese Foreign Minister Fumio Kishida told reporters when asked about the footage posted on an Arabic language Facebook account.
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But Kishida declined to comment on whether the government has identified the man on the video as Yasuda, 42.
In the video which lasts for about a minute, a bearded man wearing a black jumper with a scarf around his neck says in English: "Hello, I am Jumpei Yasuda. Today is my birthday, 16 March."
"I love you my wife, father, mother, brother... I want to hug you, I want to talk with you, but I can't anymore," he said.
"I have to say... Something to my country," he said, reading a memo as he sat at a table, a white wall to his back.
"No one is answering, no one is responding, you are invisible", he added, apparently referring to the Japanese government, adding he was "sitting in a dark room suffering with pain".
Yasuda had posted frequent comments on Twitter, expressing frustration that many journalists were staying away from Syria. But the tweets abruptly stopped on June 21 last year.
He added that unspecified "interference" with his reporting activities had increased substantially to the point that he might not be able to continue.
In the run-up to the January executions of Goto and Yukawa, the Japanese government struggled to make contact with the militants, primarily relying on countries such as Turkey as well as local religious leaders.