Jared Loughner is accused of opening fire on US congresswoman Gabrielle Giffords and a crowd of bystanders attending a political event [AFP]

The suspect in the Arizona shooting rampage that critically injured a US congresswoman has pleaded not guilty to an expanded, 49-count indictment, accusing him of killing six people and wounding 13 others.

The plea on Wednesday came shortly before the federal judge presiding over the case sided with prosecutors in ordering Jared Loughner to undergo a psychiatric evaluation to determine whether he is mentally competent to stand trial.

US District Judge Larry Burns set a May 25 hearing date on competency despite objections from defence lawyer Judy Clarke, who said such a proceeding was premature and could interfere with her ability to build trust with her client.

Loughner is accused of opening fire with a semiautomatic pistol on US congresswoman Gabrielle Giffords and a crowd of bystanders attending a political event outside a grocery store in January.

Six people, including a federal judge, were killed and 13 were wounded including Giffords, an Arizona Democrat, who was shot through the head.

In asking for a mental evaluation, prosecutors cited widely publicised accounts of erratic, paranoid behaviour by Loughner in the months before the shooting rampage, including homemade videos posted to YouTube in which Loughner talks about mistrust of the government and mind control.

The return of the new, expanded indictment against Loughner launched a formal US Justice Department review of the case to decide whether to seek the death penalty or life in prison.