BOSTON (AFP) - Shaggy-haired, bearded and attentive, accused Boston Marathon bomber Dzhokhar Tsarnaev on Thursday made his first public appearance in 17 months, attending a brief court hearing before his trial begins Jan 5.

Dressed in a black sweater and gray pants, the skinny 21-year-old with unruly curls sat between two female lawyers in a full to bursting US federal court room in the northeastern city of Boston.

He answered questions from Judge George O'Toole calmly and quietly in the brief session that lasted less than half an hour.

Tsarnaev, who faces the death penalty, is accused of carrying out the April 15, 2013 attacks that killed three people and wounded 264, the worst in the United States since the 9/11 Al-Qaeda hijackings that killed nearly 3,000.

The judge discussed leaks to the press and announced that discussions on jury selection would continue in private.

At the end, a woman started yelling out in Russian and English. "Stop killing innocent people!" she shouted.

Families of the victims, with drawn faces, were separated from the rest of the public in the gallery.

Tsarnaev has pled not guilty to 30 charges in connection with the attacks, which plunged Boston and its world-famous sporting event into mourning, and revived domestic fears of terrorism.

Before Thursday, he had not been seen in public since entering his not guilty plea in July 2013.

At the time he was suffering from injuries from his time on the run and arrest in the days after the attack. On Thursday he seemed in good health.

The charges against him include conspiracy to use a weapon of mass destruction resulting in death and bombing a public place resulting in death.

TRIAL COULD LAST MONTHS

He and his older brother Tamerlan alone are accused of planting two pressure cooker bombs hidden in back packs near the finish line of the marathon.

Tamerlan was killed in a shootout with police as the pair tried to escape the Boston area several days later. An injured Dzhokhar was captured while hiding in a boat parked in a suburban backyard.

Initially scheduled for Nov 3, his trial is now due to begin on Jan 5 with jury selection, which could take several weeks.

The trial itself could take two to three months.

Tsarnaev is being held at Fort Devens prison hospital, home to 1,095 prisoners, around 70km from the Boston court.

His lawyers say he is held largely in isolation and subject to tight restrictions imposed by Attorney-General Eric Holder in August 2013.

The special administrative measures confine him to his cell, and restrict visitors to his lawyers and immediate family, his legal team says.

They also limit his phone calls, mail and ban any contact with the media.

A Muslim of half Chechen descent, Tsarnaev emigrated with his family to the United States in 2002 and became a naturalized American in 2012.

He allegedly scrawled a rambling explanation of his motives for the Boston attacks on an interior wall of the boat.

"The US government is killing our innocent civilians," Tsarnaev wrote.

INSPIRED BY Al-QAEDA MAGAZINE

"I can't stand to see such evil go unpunished... we Muslims are one body, you hurt one you hurt us all.

"Now I don't like killing innocent people it is forbidden in Islam but... stop killing our innocent people and we will stop."

He moved to Boston aged eight with his family from Dagestan and is said to be profoundly affected by his background and early childhood in Kyrgyzstan.

The two brothers, who appear to have acted alone, prepared their bombs based on instructions in Al-Qaeda's English-language magazine Inspire, prosecutors say.

Besides families of the victims, around half a dozen supporters of the Tsarnaev brothers gathered outside court on Thursday to claim their innocence.

"I think they were set up," said Mr Josee Lepine, a retired Canadian in her 60s who came specially from Gatineau, near Ottawa.

She said the FBI had "lied" and dismissed surveillance footage that shows the brothers at the scene with their backpacks.

"I don't believe in the death penalty," she added.

US authorities are seeking a rare federal death penalty against Tsarnaev.

Legal experts, however, say the move is partly a symbolic gesture to the American public and does not necessarily mean Tsarnaev will be executed if found guilty.