North Dakota US Marshal Dallas Carlson said the incident on Monday happened after a jury returned a partial guilty verdict against the man, who had faced terrorising-related charges.

Carlson said the man had a sharp instrument that might have been made of plastic.

The jury had left the courtroom, but US District Judge Peter Welte, courtroom staff and others witnessed the incident. Court security officers and deputy marshals attempted live-saving measures in the courtroom.

FBI spokesman Kevin Smith said the man had been acquitted on one charge and found guilty on the other charge, and was about to be taken into custody.

"He did the self-harm after he heard the verdict and after the jury had left the courtroom," Smith said.

He did not know of which charge the man had been convicted.

Smith said the FBI has opened an investigation.

"We need to officially document what happened in that federal courtroom today," he said.

"All the questions you have, we have. What was the sharp object, how did it get into the courtroom, those questions, what did people see him do."

Australian Associated Press