A US Army sergeant described by former colleagues as a one-time "model soldier" was sentenced to 25 years in prison at a federal court in Hawaii yesterday after pleading guilty to providing material support to the Islamic State militant group.

Ikaika Erik Kang, 35, agreed to a plea deal in August on four counts of breaking anti-terrorism laws in which he accepted a proposed 25-year sentence. US District Judge Susan Oki Mollway accepted the terms of the plea deal during the morning's hearing, court records showed.

Kang had begun expressing support for Islamic State, which the United States deems a foreign terrorist organisation, by early 2016, according to the US Department of Justice.

Undercover agents from the Federal Bureau of Investigation began meeting with Kang earlier this year, some of them posing as members of the militant group.

Kang gave them military gear and classified documents and agreed to teach them hand-to-hand combat in a video-recorded session, according to federal prosecutors.

Lawyers for Kang filed three letters of support in court that described him as a diligent but withdrawn soldier who struggled with his mental health.

Kang's older sister, Erika Takahashi, wrote that Kang grew up in a "very abusive household," adding that he "is a good person on the inside."

Two soldiers who worked with Kang in air-traffic control at Alabama's Fort Rucker military post used their letters to urge the judge to help Kang get counselling.

Thomas Maia, who was Kang's first supervisor at Fort Rucker, called Kang a "model soldier" but said he had worried about Kang's odd behaviour. This included staring at a wall for hours on end, saying he was trying to listen to the sound of his blood running through his veins, Maia wrote.

Maia wrote that his efforts to secure a mental health evaluation for Kang were rebuffed.

"He didn't seek out ISIS on his own, he was approached and socially engineered by the FBI at the Army's request," Maia wrote, using an acronym to refer to the Islamic State. "If he would have been given adequate mental treatment back when I asked for it, none of this would have happened."

Federal prosecutors say Kang agreed to swear an oath of loyalty to Islamic State in a pseudo-ceremony organised by the undercover FBI agents. After the ceremony, Kang told the agents he was ready to take his rifle to downtown Honolulu and start shooting, whereupon he was arrested.

– Reuters