LOS ANGELES (AFP) - A US teenager pleaded guilty Wednesday to trying to help the Islamic State in Iraq and Syria (ISIS), in a deal with prosecutors requiring her to tell all she knows about the jihadist network.

Shannon Maureen Conley, 19, was arrested in April as she prepared to fly to Turkey to join a jihadist whom she had agreed to marry, and who was fighting in Syria.

She was charged in July, but on Wednesday prosecutors announced a plea deal in which she admitted conspiracy to provide material support to a designated foreign terrorist organisation. In exchange prosecutors agree not to charge her with any other criminal activity, according to the plea deal.

She also agreed to "cooperate and debrief (investigators) completely and truthfully... concerning her knowledge of other individuals involved in providing or attempting to provide material support... to any terrorist organization," it said.

Conley was described as a quiet girl, but changed markedly in the six months before her arrest, after meeting the fighter, a 32-year-old Tunisian identified only as "Y.M.," according to court documents.

A certified nurse aide, she allegedly agreed to get engaged to him and join him in Syria, where she offered to provide any help she could, whether as a nurse or otherwise. "Y.M." was with ISIS, which is backed by thousands of fighters and has captured swathes of Iraq in its drive to create an "Islamic caliphate".

In preparation for her mission, Conley joined the US Army Explorers (USAE) last September to be trained in US military tactics and firearms, it is alleged. She was arrested at Denver airport on April 8 as she prepared to board a flight for Istanbul, via Frankfurt.

Conley was initially charged with conspiracy to provide material support to a designated foreign terrorist organization. If convicted, she had faced up to five years in jail or a US$250,000 (S$316,000) fine, or both.