The bearded man, in an orange jumpsuit, is kneeling, surrounded by two hooded gunmen in black. In the video shown on various media websites, he says in Japanese in a strained voice, “My name is Umar. I am South Korean. Today’s date is July 25, 2018. I am in a terrible situation. Please help me immediately.”

It was not clear why he identified himself as Korean or used that name.

Japan's top government spokesman said he believed the man was Yasuda.

"It is the government's prime responsibility to protect Japanese, and we are making the utmost efforts, using various information networks," Chief Cabinet Secretary Yoshihide Suga told a regular news conference. He declined to comment further.

A Foreign Ministry official said the government believes Yasuda holds Japanese nationality.

Yasuda was reported by Japanese media as being captured by an al Qaeda affiliate after entering Syria from Turkey in 2015. Since then, he has appeared occasionally in online videos, his hair greying, lengthening and becoming more unkempt.

Japanese would-be soldier of fortune Haruna Yukawa was captured by the self-data-styled Islamic State after slipping into Aleppo in August 2014. His friend, journalist Kenji Goto, sought to help Yukawa and was also captured. Both were beheaded in early 2015.

REUTERS