KEY POINTS:

• 12 of 17 dead in Valentine's Day shooting at Marjory Stoneman Douglas High School were killed inside school buildings
• Gunman identified as former student Nicolas Cruz, 19, expelled last year
• Evacuated students tell of walking past "motionless bodies"
• Gunman believed to have used an AR-15 rifle

At least 17 people are dead and many more are injured in a Florida high school shooting on what is Valentine's Day in the US.

The gunman, who reportedly used an AR-15 rifle, was captured by police at a house barely a kilometre away from Marjory Stoneman Douglas High School where the shooting happened shortly before dismissal time.

He has been identified as former student Nicolas de Jesus Cruz, 19, whose Instagram page showed disturbing images of him posing with multiple types of firearms and weapons. He was expelled last year for disciplinary issues, authorities have confirmed.

Broward County sheriff Scott Israel said 12 of the 17 dead were killed inside the school's buildings. two died on the campus and one was killed at a nearby intersection.

Medical officials for the Broward Health System said two people have died in hospitals, three are in critical condition and three are stable.

Casualty numbers are expected to rise.

"There are fatalities involved here," school superintendent Robert Runcie said. "It's just a horrible day for us."

Florida senator Bill Nelson told local media that "many people have been killed" in the shooting at the 3000-pupil school.

A student told media that classmates hid in a closet at the school when the shooting started.

"All of a sudden there was a really loud noise… people are crying in the closet," the student wrote to CBS 4.

Parent John Obin says his son, a freshman at the South Florida school where the shooting erupted, was in class when he heard several shots.

Obin says teachers quickly rushed students out of the school. He said his son told him that he walked by two people on the ground motionless — apparently dead — as students rushed outside.

"This is a really good school, and now it's like a war zone," Obin said.

An unidentified student told a reporter that at first students thought it was a fire drill because they had heard fire alarms going off.

"All of a sudden there was a really loud noise… people are crying in the closet," the student wrote to CBS 4.

Others said they thought it was balloons popping, as many students had them to mark Valentine's Day.

The shooter was described as wearing a black hat, a maroon or burgundy shirt and black pants. He was apprehended at a house about a kilometre from the school.

Television footage showed police putting a person in the back of a police car outside the high school.

Police with guns drawn swarmed the school shortly before lessons finished for the day, WPLG reported. The school was on lockdown as students fled, carrying their bags. The lockdown has now been lifted.

Some students were seen lined up in single file, leaving the area in an orderly fashion. Other students were hysterical.

Runcie said the school had no warning of the shooting, adding: "Mental health issues in this country are growing."

However, a student at the school has told WFOR-TV that all his classmates "knew" the shooter would be Cruz.

"She's terrified," the mother said. "She said she heard the gunshot."

"I'm glad that she's able to text," she added.

Another mother told the affiliate her daughter "keeps telling me to stay away. 'Be safe, Mom, stay away.'

"I won't stay away," she said, telling her daughter, "I'm your mother."

President Donald Trump has been briefed on the shooting, and has tweeted his "condolences" to the families of the victims, calling the shooting "terrible".

"No child, teacher or anyone else should ever feel unsafe in an American school."

It is at least the 19th school shooting in the US this year, according to gun control advocacy group Everytown For Gun Safety.