LOS ANGELES (AFP) - A United States court on Friday jailed a "radicalised" Colorado teenager intercepted by the FBI when she attempted to join her fiance in the Islamic State in Iraq and Syria (ISIS) group to fight in Syria.

Shannon Conley, 19, will serve four years in prison for providing and attempting to provide material support and resources to ISIS fighters and other extremist groups including Al-Qaeda, the US justice department said.

Conley, who struck a plea bargain with prosecutors, had expressed a desire to wage holy war after meeting a man on the Internet who claimed to be an active member of ISIS in Syria. The duo got engaged and worked together to have Conley travel to Syria to join her new fiance.

Before going, Conley trained to be able to fight and even joined the US Army Explorers (USAE) to be learn about military tactics and firearms. She also had first-aid training. FBI special agents met her several times to persuade her not to carry out her plans to travel overseas to fight but she refused to listen and was arrested when she attempted to board a flight to Turkey from Denver on April 8.

A search of Conley's home revealed books and articles about terrorist groups.

"The defendant in this case got lucky," said US attorney John Walsh.

"The FBI arrested her after determining that she had been radicalised and planned to travel to Syria to support the brutal foreign terrorist organisations operating there.

"Had she succeeded in her plan to get to Syria, she would likely have been brutalised, killed or sent back to the United States to commit other crimes."

After prison, Conley will have three years on supervised release, followed by 100 hours of community service.