LOS ANGELES (AFP) - A US teenager who agreed to marry a fighter from the militant network that is threatening to overrun Iraq has been arrested and charged with trying to help a foreign terror group.

Shannon Maureen Conley, 19, was arrested in April as she prepared to fly to Turkey to join the jihadist, who was fighting in Syria, according to court documents unsealed this week.

She 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." Conley, a registered nurse, 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 the Islamic State of Iraq and the Levant (ISIL), which has captured swathes of Iraq in its drive to create an "Islamic caliphate."

ISIL is backed by thousands of Islamist fighters in Syria and Iraq.

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.

"Conley said she planned to tell her family about her jihad plans once she was out of the US as there was nothing they would be able to do about it then," said an affidavit released by the Colorado US Attorneys Office.

Asked if she would be interested in working for an aid organisation or charity, "Conley stated she has no interest in doing humanitarian work," it said.

"Conley felt that jihad is the only answer to correct the wrongs against the Muslim world. Conley said she preferred to wage jihad overseas so she could be with jihadist fighters."

She was arrested at Denver airport on April 8 as she prepared to board a flight for Istanbul, via Frankfurt.

Conley is charged with conspiracy to provide material support to a designated foreign terrorist organisation.

If convicted, she faces up to five years in jail or a US$250,000 (S$311,857) fine, or both.