NEW YORK A Somali teenager accused of leading a pirate attack on an American cargo ship off the coast of Africa pleaded not guilty Thursday to piracy charges. A somber-looking Abdiwali Abdiqadir Muse entered the plea in a soft voice with his head down during a brief appearance in U.S. District Court in Manhattan. Afterward, his lawyers said Muse has been kept in solitary confinement since being captured in his impoverished nation and brought to the United States last month a predicament that's left him scared and confused. 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 magistrate judge concluded he could be tried as an adult since there was evidence that he is 18. Authorities say Muse was the only surviving pirate among a group that attacked the U.S.-flagged Maersk Alabama on April 8. He was captured April 12 as U.S. military snipers fired on a group holding the cargo vessel's captain hostage. The captain, Richard Phillips, has since returned to his home in Underhill, Vt., to a hero's welcome. Muse was indicted earlier this week on 10 counts including piracy under the law of nations, conspiracy, hostage taking, kidnapping and possession of a machine gun while seizing a ship by force. If convicted of the most serious charge, he faces a mandatory life sentence. When the Maersk Alabama was attacked, it was carrying humanitarian supplies about 280 miles off the Somali coast. U.S. authorities say Muse led the attack on the ship, firing his AK-47 assault rifle at Phillips. Authorities say Phillips, held hostage for several days on a lifeboat, had an AK-47 held near his back when the snipers killed three pirates as the small vessel bobbed in the water near three U.S. warships and beneath a helicopter. Muse was ordered to return to court on Sept. 17.