US authorities have arrested three Islamic State sympathisers who allegedly plotted to join the jihadist group or, alternatively, carry out terror attacks on American soil.

The three, all men of central Asian descent living in Brooklyn, have been charged with conspiring to support Isis.

The FBI said they planned to travel to Syria via Turkey to fight for the extremist group there.

"The defendants looked to join the Islamic State of Iraq and the Levant [Isis], by flying to Turkey in a vain attempt to evade detection," FBI Assistant Director in Charge Diego Rodriguez said in a statement.

"And several of the defendants planned to commit acts of terror here - in America - if they could not travel, to include killing FBI agents."

The suspects have been identified as, Akhror Saidakhmetov, 19, of Kazakhstan, Abdurasul Hasanovich Juraboev, 24, of Uzbekistan and Abror Habibov, 30, also of Uzbekistan.

Saidakhmetov was held this morning at New York's John F. Kennedy International Airport as he was about to take a plane to Istanbul.

In a conversation taped by police, he threatened to purchase a Kalashnikov rifle and embark on a killing spree against security officers if he was not allowed to leave.

"I will just go out and buy a machine gun, AK-47, go out and shoot all police," he said according to court documents, quoted by the CNN.

The other two were arrested in Brooklyn and in Florida.

Sidakhmetov and Juraboev first came to the attention of authorities this summer as they showed their support for Islamist group responsible for the beheading of several US hostages online.

Juraboev wrote on an Uzbek-language website he was even ready to murder President Barack Obama if instructed to do so by Isis, CBS reported.

Habibov has been accused of helping fund Saidakhmetov's trip to Syria, Reuters reported.

The three are due to appear before a federal court later today.