(Reuters) - U.S. officials on Tuesday agreed to loosen prison restrictions on accused Boston Marathon bomber Dzhokhar Tsarnaev that the 20-year-old's lawyers have argued made it difficult to mount an effective defense.

In papers filed in U.S. District Court in Boston, officials said they had modified the special rules of Tsarnaev's confinement to allow his attorneys to discuss his statements with third parties to help them prepare their defense.

The new rules also allow additional people, including a mental health specialist, to meet with him without an attorney present.

Tsarnaev is accused of planting two homemade pressure-cooker bombs at the crowded finish line of the marathon on April 15, killing three people and injuring more than 260.

Prosecutors contend that Tsarnaev and his older brother, Tamerlan, killed a university police officer three days later in an unsuccessful attempt to steal his weapon as they prepared to flee the city.

Tamerlan, 26, died later that night in a gun battle with police. Dzhokhar got away, prompting a daylong lockdown of most of the Boston area as police searched for him. Dzhokhar was found hiding in a dry docked boat in a backyard in Watertown, Massachusetts, on April 19.

He has pleaded not guilty to all charges. If convicted, he faces the possibility of execution.

Tsarnaev is kept separate from other inmates at the prison facility west of Boston where he is being held awaiting trial.

(Reporting by Scott Malone in New York; Editing by Colleen Jenkins and Gunna Dickson)