An Israeli official has claimed that nine Japanese nationals have joined the ranks of the Islamic State (also known as Isis) in the Middle East, according to Japan's former air force chief Toshio Tamogami.

Tamogami maintained on his blog that Nissim Ben Shitrit, director-general of Israel's foreign ministry, confided to him earlier in September that the Japanese citizens travelled to Syria and Iraq to wage jihad.

The news was not confirmed by the Japanese government. The former military boss, now a senior official of a small political party, said the Israeli official declined to elaborate further on the Japanese would-be jihadists. "He was tight-lipped," he said, according to Reuters.

It is not the first time that Japanese people are said to be present in Syria. A Japanese man believed to be a mercenary operating in Syria was filmed being tied up and interrogated by Isis jihadists. The man, named Haruna Yukawa, is believed to have travelled to the Middle East for his job as adviser for the PMC security firm.

Kimoto, a regional representative of nationalist group Gambare Nippon (Stand Firm, Japan) also confirmed Yukawa was active in the group.

The development came after an al-Qaida-linked Filipino terror group, Abu Sayyaf, threatened to execute two German hostages it kidnapped in April unless Germany pays a ransom and cease its support to US-led air strikes against Isis.

The Islamist militants are holding about 10 hostages in the Sulu jungles, including two European birdwatchers, a Japanese national and some Filipinos, according to AP reports.