By Tom Perry and Fiona Ortiz

BEIRUT/CHICAGO (Reuters) - A U.S. citizen has joined Kurdish forces fighting against Islamic State militants in northern Syria, a spokesman for the main Kurdish armed group in the country said on Thursday.

The Kurdish official said that Jordan Matson had joined the Kurdish People's Protection Units (YPG), who are mainly battling advances by Islamic State close to Syria's borders with Turkey and Iraq.

"Yes it is true," YPG spokesman Redur Xelil said in an online message. "He is fighting in the Jazaa area."

Jazaa is a town in Syria's northeastern Hasaka province, close to the Iraqi border and has been the site of heavy fighting between the two groups.

The YPG said last month it has lost 35 of its fighters in a two-week battle for control of Jazaa and said Kurdish forces had killed hundreds of Islamic State fighters.

On Thursday Islamic State tightened its grip on a town further west, close to the border with Turkey despite coalition air strikes meant to weaken the group. The fighting has sent tens of thousands of refugees into Turkey since last month.

Xelil, who said he had met Matson, sent a link to online photos on a Kurdish news agency which he said were of the American.

They show a smiling young man in an army shirt with a white and black scarf wrapped around his head. Xelil declined to give further details.

A Facebook page for Jordan Matson says he attended Case High School in Racine, Wisconsin. Reports on social media have said he is 28 years old.

A friend of Matson's said he told online gaming friends about two months ago that he was joining a "private army" to fight Islamic State.

"He told us in the community that he was getting hired by a private army and he let us know two to three months in advance," said Miguel Caron by telephone from Montreal.

"He sent me a personal Facebook message on the 16th of September saying 'hey boss, I'm heading to Syria.' He told me he dropped his girlfriend and stopped looking for a job," Caron added.

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Caron, who works for a video and online game company, said he had met Matson at a trade show and that Matson had said he was formerly in the military.

Caron said Matson had posted on Facebook this week that he was in Syria and had been wounded in fighting, but not seriously. The Facebook post has been disseminated via Twitter messages, but is only visible to Matson's friends on his Facebook page.

(Editing by Dominic Evans)