(Updates with release)

NABLUS, West Bank, Oct 11 (Reuters) - A previously unknown Palestinian armed group briefly kidnapped an American student volunteer in the West Bank on Wednesday but released him within hours and unconditionally, a Palestinian security source said.

Michael Phillips, 24, was returned to his lodgings in the city of Nablus unharmed after local Palestinian officials intervened to secure his release, the source said.

"I'm fine, and I'm happy that this incident is over," Phillips told Reuters by telephone. Asked if he had been mistreated by his captors, he said: "Nothing like that happened."

A group calling itself Ansar Assuna seized Phillips at around 5 p.m., faxing Reuters a copy of his passport, Israeli entry visa, and student card. The group had no known links to Palestinian militants waging a 6-year-old revolt against Israel.

In a later statement, Ansar Assuna said the abduction sought to "confront the American war on Islam" and demanded the release of Palestinian women and minors imprisoned in Israel.

Phillips works as an English teacher with a pro-Palestinian charity in Nablus. His abduction drew condemnation from the U.S. State Department. The kidnapping follows the abduction of two journalists working for the American Fox New Channel in the Gaza Strip in August. They were held by militants for two weeks before being freed.

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