Parkland school shooting: US prosecutor seeks death penalty for gunman Nikolas Cruz

A prosecutor on Monday told jurors they should sentence to death the gunman who killed 17 people and wounded another 17 in a mass shooting at a Florida high school in 2018.

Nikolas Cruz pleaded guilty in October to the premeditated murder of 14 pupils and three staff members at Marjory Stoneman Douglas High School in Parkland, about 48 kilometres north of Fort Lauderdale.

Prosecutor Michael Satz told Broward County jurors on the first day of the penalty phase of the trial that Cruz committed "goal-directed planned, systematic murder — mass murder — of 14 students, an athletic director, a teacher and a coach".

Mr Satz called witnesses, including pupils who were at school that day, and played mobile phone videos in which terrified students cried for help or spoke in whispers as they hid.

About three dozen family members of the victims were in court, some of whom cried as they watched the proceedings.

Dressed in a gray-and-black jumper and wearing a black mask, Cruz sat with his head in his hands, at times resting it on the defence table, as the videos were played.

Cruz, who was 19 and an expelled pupil at the time of the massacre, will be sentenced to life in prison without parole if any of the 12 jurors objects to the death penalty. Their decision could take months.

In his guilty plea, Cruz said he was "very sorry" and asked to be given a chance to help others.

Mr Satz said aggravating factors in the case, including premeditation, outweighed arguments for leniency, including Cruz's history of mental health problems.

Pupil Dylan Kraemer was in a Holocaust history class on that day when he heard a loud noise.

"Eventually the shooter started shooting through the window, bullets were flying through," Dylan said.

"We ducked down, waited 20 or 30 seconds, it stopped. I looked over and two people were dead, multiple people were shot."

Teacher Brittany Sinitch described calling the 911 emergency number from her classroom.

Ms Sinitch's pupils were writing Valentine's letters as characters from Romeo and Juliet as the attack began on the afternoon of February 14, 2018.

"Almost instantly, I called 911. They couldn't hear me over the sound of the gunshots, it was so loud," she said.

Lawyers for Cruz were expected to make their opening arguments later in proceedings.

US gun violence has had renewed attention after recent mass shootings including one at an Independence Day parade outside Chicago that killed seven people, and another in May at a primary school in Uvalde, Texas, that killed 19 children and two teachers.

President Joe Biden in June signed the first major federal gun reform in three decades, which he has celebrated as a rare bipartisan agreement.