CENTENNIAL (AFP) - Prosecutors in the US state of Colorado said on Tuesday that they will seek the death penalty against the man accused of killing 12 people in a crowded cinema in July last year.

"For James Holmes in this case, justice is death," district attorney George Brauchler told a court in Centennial, where the 25-year-old Holmes is expected to go on trial in August.

Mr Holmes is accused over the July 20 massacre at a midnight screening of the Batman movie "The Dark Knight Rises" in Aurora, Colorado, which revived America's long-running debate about gun control.

Witnesses said Mr Holmes threw smoke bomb-type devices before opening fire randomly with weapons, including an AR-15 military-style rifle, a 12-gauge shotgun and a .40-caliber pistol.

Last week, defence lawyers filed an unexpected motion offering that Mr Holmes would plead guilty, in return for the prosecution not pressing for the death penalty, but agreeing to a sentence of life in prison without parole.

But prosecutors shot back within 24 hours, accusing the defence of trying to negotiate a plea deal in public, in violation of a gag order on the horrific case.