WATCH: SWAT team storms classroom during school shooting

WARNING, DISTURBING CONTENT: A SWAT team has been filmed storming a US classroom where terrified students huddled in fear of a gunman who killed at least 17 people and injured dozens more.

Covered in bulletproof vests and helmets, several heavily armed SWAT officers made their way into a locked down classroom at Marjory Stoneman Douglas High School in Parkland, Florida.

One female student can be seen trembling in absolute fear as the officers move towards them.

Students were quick to follow orders and raise their hands in the air before another officer instructed them to put their phones away.

The video was shared by the sister of one of the students in the classroom.

"My little brother just sent me this video of the swat team evacuating his classroom at Stoneman Douglas," Melody Ball wrote on Twitter.

"So scary but glad he’s safe."

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Nikolaus Cruz, a former student of the school, was arrested about an hour after the shooting rampage.

Twelve of the dead were killed inside the school building, two others just outside, one more on the street and two other victims died of their injuries at a hospital, Broward County Sheriff Scott Israel said.

Staff and students told local media that a fire alarm went off around the time the shooting started, sparking chaos as some 3,000 students at the school first headed into hallways before teachers herded them back into classrooms, to seek shelter in closets.

Authorities at two nearby hospitals said they were treating 13 survivors for bullet wounds and other injuries, five of whom were listed in critical condition.

The Valentine's Day bloodshed in the Miami suburb of gated communities with palm- and shrub-lined streets was the latest outbreak of gun violence that has become a regular occurrence at schools and college campuses across the United States over the past several years.

It was the 18th shooting in a US school so far this year alone, according to gun control group Everytown for Gun Safety.