Birthday girl, 3, dies after being stabbed at her party

Timmy Kinner, a Los Angeles resident, has been charged. [Reuters]

A three-year-old girl died of her injuries on Monday, after a knife attack at her birthday party left nine injured in Boise, Idaho, US police say.

They believe Saturday's attack at a complex, home to many refugees, was motivated by "vengeance" after the suspect was asked to leave the place the day before following bad behaviour.

Seven people remain in hospital, some with "serious or critical injuries".

The suspect has been named as Timmy Earl Kinner, a US citizen.

All nine people attacked on Saturday are members of refugee families from Ethiopia, Iraq and Syria.

Six children - aged between three and 12 - were injured, including the three-year-old birthday girl who died in Utah after being flown there for treatment.

Police said on Monday that one child has been treated and released from hospital.

Police chief William Bones said: "This was an attack on those who are most vulnerable: our children."

He described the attack, in a statement, as "untenable, unconscionable" and "pure evil".

Mr Kinner, 30, is charged with first-degree murder, several counts of aggravated battery and six counts of injury to a child.

The Los Angeles resident wanted to represent himself in court, the Associated Press reported, but a judge ordered a public defender.

Mr Kinner, who has an extensive criminal history spanning several states, according to police, is being held without bail.

Mr Bones told reporters that the evidence did not suggest the attack was a hate crime.

The suspect had been staying with an acquaintance at the complex - but was kicked out on Friday due to bad behaviour.

He returned the following day to "exact vengeance", attacking people at the birthday party, the police chief said.

"It appears from the investigation, it was a matter of where they [victims] lived, not a matter if they knew the suspect," Mr Bones added.

Boise Mayor Dave Bieter said, in a statement, that he was "heartbroken" to learn of the child's death.

"The thought of a happy occasion like a child's birthday party being turned into the darkest of sorrows in such a vicious way is almost unfathomable."