Two college friends of accused Boston Marathon bomber Dzhokhar Tsarnaev have pleaded not guilty to charges of obstruction of justice and conspiracy.

The two students are accused in federal court of removing a backpack containing fireworks and a laptop computer from Mr Tsarnaev's room at the University of Massachusetts Dartmouth as the FBI was searching for him.

Dias Kadyrbayev and Azamat Tazhayakov, who are both 19 and from Kazakhstan, could face 25 years in prison or deportation if convicted.

Neither is charged with involvement in the bombing.

Mr Tsarnaev, 20, is accused of setting off two homemade pressure-cooker bombs at the race's crowded finish line on 15 April.

Three people were killed and 264 injured in what was one of the worst attacks on US soil since 11 September, 2001.

He is also charged in the death of a university police officer from whom he and his older brother, Tamerlan, tried unsuccessfully to steal a gun, according to authorities.

An attorney for Mr Kadyrbayev said his client cooperated with investigators from the moment they approached him, turning over Mr Tsarnaev's computer and telling the FBI where it could find the backpack.

"Dias comes from a former Soviet-bloc region where police routinely are distrusted," attorney Robert Stahl told reporters.

“Yet when authorities first approached him, he fully cooperated and for nearly 12 hours over two days Dias answered the FBI's questions without an attorney or a Kazakh consular official present."

Mr Tazhayakov's attorney, Arkady Bukh, said his client had not given Mr Kadyrbayev consent to throw the bag away.

Mr Stahl said he hoped Mr Kadyrbayev would be acquitted and allowed to return to his family in Kazakhstan.

The two have been in federal custody since 1 May, when lesser charges of conspiracy were filed against them.

They were indicted on the obstruction of justice charges on 8 August.

A third friend, Robel Phillipos of Cambridge, Massachusetts, is engaged in talks with prosecutors aimed at settling a charge of making false statements in a terrorism investigation.