FBI agents escort Abdiwali Abdiqadir Muse into FBI headquarters in New York on April 20. (Louis Lanzano/Associated Press)

A Somalian teenager accused of leading a pirate attack on an American cargo ship off Somalia's coast last month pleaded not guilty to piracy on Thursday in a U.S. court.

Abdiwali Abdiqadir Muse faces life in prison if convicted on charges of piracy and other offences in a Manhattan federal court.

Muse was taken into custody aboard a U.S. navy ship off the coast of Somalia on April 12, shortly before Navy SEAL snipers killed three other pirates holding Capt. Richard Phillips hostage on a lifeboat launched from his cargo vessel, the Maersk Alabama.

Muse's lawyers said he is terrified after being held in solitary confinment since arriving in the U.S. They continued to deny the U.S. government's claim that their client is 18.

"He's a boy who fishes and now he's ended up in solitary confinement," said one of the lawyers, Deirdre von Dornum. "It's truly terrifying."

Muse's age has been in dispute since his arrest. His family has said he is as young as 15, but a U.S. judge concluded he could be tried as an adult since there was evidence that he is 18.

Phillips was hailed as a hero upon his release for surrendering himself to the heavily armed pirates to save his crew, who thwarted the hijacking.

In addition to piracy, Muse is charged with conspiracy to seize a ship by force; discharging a firearm; aiding and abetting the discharge of a firearm during a conspiracy to seize a ship by force; conspiracy to commit hostage-taking; and brandishing a firearm.

With files from The Associated Press