A tech-savvy US teenager who was prolific on Twitter pleaded guilty Thursday to using social media to aid the Islamic State group, a crime punishable by up to 15 years in prison.

Ali Shukri Amin, 17, who sent more than 7,000 Twitter messages in support of IS, admitted to providing advice and encouragement to the extremist outfit and its supporters.

"Guilty, sir," the slender high-school student, sporting the trace of a moustache, answered when asked by the judge in the court in Alexandria, Virginia to state his plea to the charges.

His lawyer, Joseph Flood, said the case was unprecedented in that Amin was the first minor to be convicted in the United States of providing material aid to IS.

"He's deeply remorseful," Flood told reporters after the hearing, adding that his client has been cooperating with law enforcement.

Under the Twitter handle @Amreekiwitness, Amin provided IS supporters with instructions on using the virtual currency Bitcoin to conceal financial donations to the radical Islamist group and the best way to encrypt their online exchanges.

The teenager also expressed support for jihad on his Al-Khilafah Aridat blog.

And he used what the FBI official leading the investigation called a "prolific online presence" to offer guidance to supporters seeking to travel to Syria to fight with IS.

The Justice Department said Amin facilitated travel for another young IS recruit, Reza Niknejad, 18, who traveled to Syria to join the jihadist group in January.

Niknejad was charged Wednesday in federal court in Virginia with conspiring to provide material support to terrorists, conspiring to provide material support to IS and conspiring to kill and injure people abroad.

Flood described Amin as a stellar student from a good family who was outraged by rights abuses under Syria's President Bashar al-Assad.

"Amin has sincere religious beliefs that led him into activity that violated US law," Flood said.

"He had a bright future ahead of him and he's made some bad decisions that he's taking adult responsibility for."

- IS message reaching America -

John Carlin, US Assistant Attorney for National Security, urged parents to monitor what their children are doing online.

"Around the nation, we are seeing ISIL (IS) use social media to reach out from the other side of the world," he said.

"Their messages are reaching America in an attempt to radicalize, recruit and incite our youth and others to support ISIL's violent causes."

He added: "This case serves as a wake-up call that ISIL's propaganda and recruitment materials are in your communities and being viewed by your youth.

"This challenge requires parental and community awareness and action to confront and deter this threat wherever it surfaces."

Separately, a federal court in New York unsealed an indictment charging Akmal Zakirov with attempt and conspiracy to provide material support to the IS group, in concert with four other suspects who already have been charged.

Zakirov faces up to 30 years in prison if convicted.