Japan said on Thursday it was investigating footage of a man identifying himself as a Japanese journalist believed missing in Syria, who appeared to be speaking under duress and seeking the government's help.

The government is sensitive to any news of possible Japanese captures abroad after Islamic State militants claimed to have beheaded war correspondent Kenji Goto last year, a week after the group also said it killed Haruna Yukawa, a self-styled contractor.

The video of Jumpei Yasuda was widely shown on Japanese TV news programs on Thursday.

It was posted online by a Syrian man who lives in Turkey who said Yasuda was taken hostage by the al-Nusra Front rebel group in Syria, public broadcaster NHK and the Asahi Shimbun daily reported, citing telephone conversations with the man.

Speculation in Japan has swirled around Yasuda since last year amid fears he may have been captured in Syria. The Japanese government said in December that it was investigating.

"We are aware of the video and are analyzing it," Japanese Foreign Minister Fumio Kishida told reporters on Thursday when asked about the footage posted on an Arabic language Facebook account.

One-minute video

But Kishida declined to comment on whether Tokyo 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, March 16."

His bearing was calm but he occasionally paused to show emotion.

"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.

In his last Twitter post on that day, he said: "I have reported what is happening through my blog without disclosing where I am."

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.