Haruna Yukawa Twitter TOKYO (Reuters) - Japan is investigating information that one of its citizens has been captured in northern Syria by the Islamic State militant group, the foreign ministry said, as evidence emerged that the captured man appeared to be a self-styled security contractor.

A video clip posted on YouTube showed a T-shirt clad man lying on the ground being questioned by unidentified persons and responding that he was Japanese and that his name was Haruna Yukawa. He also said he was part-journalist, part doctor.

The name is the same as that of a chief executive of a self-described private mercenary and security firm. No one answered the telephone at the Tokyo-based company.

In the video clip, the authenticity of which could not be independently verified, the man can be heard being asked in English, "Why do you have a gun?" But his answer is inaudible.

More than 170,000 people have been killed in Syria's civil war, which pits overwhelmingly Sunni Muslim rebels against President Bashar al-Assad, a member of the Shi'ite-derived Alawite minority, backed by Shi'ite militias from Iraq and Lebanon.

"This is an area where a various groups are carrying out battle. We are focusing on confirming if such capturing has really taken place, as well as the safety of the captured," a Japanese foreign ministry official told reporters. The official added the ministry was not aware that any group had claimed responsibility for the capture or of any ransom demand.

A Facebook posting by the head of the Japanese security firm, PMC (Private Military Company) Co Ltd, on July 11 showed him test firing an assault rifle in what he says is Aleppo, Syria. His Facebook page also showed pictures purporting to be from the Iraqi border. In a series of pictures, he poses in an armored vehicle and complains of the heat.

The firm was set up in January and according to its filing with the Japanese government, engages in a range of business from website design and pet food sales to security services.

An adviser to the firm, Nobuo Kimoto, told Reuters that Yukawa had traveled to Syria earlier this year "because he thought it would be useful for his work". Kimoto, a regional representative of nationalist group "Gambare Nippon" (Stand Firm, Japan) also said Yukawa was active in the group.

The conflict in Syria started when Assad cracked down on a pro-democracy uprising, which then armed itself.

Until this summer, Assad's forces held off from targeting Islamic State, an al Qaeda offshoot formerly known as the Islamic State in Iraq and the Levant (ISIL).

This has allowed the group to thrive and also weaken less hardline opposition groups that are backed by the West.

Assad has long painted the uprising in Syria as a foreign-backed Islamist conspiracy and his enemies say he has allowed the Islamic State to grow to promote that idea.

But this month, Islamic State fighters have gained momentum in Syria, boosted by equipment seized in a rapid offensive in neighboring Iraq, and the Syrian army has become more confrontational, using air strikes to kill fighters.

PMC adviser Kimoto said he had lost contact with Yukawa around mid-July. Japan's Kyodo news agency, quoting an unidentified local leader of rival militant group the Islamic Front, reported that Yukawa had been traveling with them and had gone to report on the conflict after entering Syria on July 28.

(Additional reporting Olivier Fabre, Antoni Slodkowski and Kevin Krolicki; Writing by Linda Sieg; Editing by Clarence Fernandez and Jeremy Laurence)