Footage has emerged of a Japanese journalist who went missing in Syria apparently asking for Tokyo's help in securing his release.

The Japanese government said Thursday it was studying the video of Jumpei Yasuda, who has not been heard from since the middle of last year.

The footage was posted online by a Syrian man who lives in Turkey, who said Yasuda had been taken hostage by the al-Qaeda-linked Nusra Front rebel group in Syria, NHK and the Asahi Shimbun daily reported.

They cited telephone conversations with the man, though did not identify him.

Broadcaster Nippon TV also said it spoke by phone with a person it described as a negotiator for the group, and that it is seeking a ransom.

Chief Cabinet Secretary Yoshihide Suga, the government's top spokesman, said that he was "not aware" of any ransom demand.

Speculation has swirled around Yasuda since reports first emerged that he may have been captured in Syria.

"We are aware of the video and are analysing it," Japanese Foreign Minister Fumio Kishida told reporters on Thursday.

In the one-minute video, the 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."

He remains mostly calm, but becomes slightly emotional when he addresses a message to members of his family.

Speaking "to my country", Yasuda indicates there has been no official response to his plight.

Militants from the self-styled Islamic State group last year beheaded Japanese war correspondent Kenji Goto and his friend Haruna Yukawa.

The government in Tokyo was criticised for what detractors saw as its flat-footed response to the crisis at the time, including apparently missed opportunities to free both men.

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.

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

He added that unspecified "interference" with his reporting activities had increased substantially to the point that he might not be able to continue.