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The man accused of the Arizona shooting attack that killed six people and wounded 13, including a US congresswoman, has pleaded not guilty to three attempted murder charges.

Jared Loughner, 22, entered the pleas over the attempted assassination of Representative Gabrielle Giffords and two aides.

State charges in the six deaths and other injuries are expected to follow.

Ms Giffords, who was shot in the head, is at a rehabilitation centre in Texas.

Mr Loughner, who has been in jail since the attack, arrived amid high security for his arraignment hearing.

Wearing glasses and an orange prison jumpsuit, Mr Loughner sat quietly through the whole hearing, smiling broadly, the Associated Press reported.

'Remarkable progress'

The 8 January shooting occurred at a meeting Ms Giffords, a Democrat, was holding for constituents at a supermarket in Tucson, Arizona.

Among the dead were a nine-year-old girl and federal judge John Roll. Federal prosecutors are weighing whether Mr Loughner can be charged with a federal crime over his death.

Prosecutors have said Mr Loughner, who had been rejected by the US military due to drug use and suspended from a college amid concerns about his mental health, targeted Ms Giffords for assassination.

Ms Giffords has made what doctors call remarkable progress, and is undergoing a course of intensive rehabilitation at Memorial Hermann Rehabilitation Hospital in Houston, where her husband Mark Kelly works as a Nasa astronaut.

Mr Loughner is represented by Judy Clarke, a prominent defence lawyer who represented an infamous parcel bomber and the man convicted of setting a bomb at the 1996 Olympic games in Atlanta. She is known as a fierce opponent of capital punishment.