Deputies in Broward County, Florida, responded to an active shooter situation at Marjory Stoneman Douglas High School on Wednesday afternoon local time, which was placed on a "code red" lockdown.

The suspect, a 19-year-old former student at the school, has been taken into custody after he was found off-campus, local police said.

Police have confirmed 17 people have been killed in the shooting, with more undergoing surgery in hospital.

"It's a horrific situation, it's just a horrible day for us."

The school superintendent said the alleged shooter was a former student of the school, and there was no evidence there had been more than one shooter.

 "We're going to do whatever we can to come together as a community and pull through this, and we will," Superintendent Runcie said.

"We're going to do whatever we can to come together as a community and pull through this, and we will," Superintendent Runcie said.

At least 14 people were injured in the shooting, and Broward County police say victims continue to be transported to local hospitals.

Broward County Sherriff Scott Israel said the alleged shooter was found off the school campus and was taken into custody "without incident".

"There was a time when he did attend the school, we don't know why he left," he said.

The Sherriff said the scene was still active as SWAT teams and law enforcement worked to ensure all students and staff were accounted for.

"We believe at this point that all children are cleared ... [but] we don't know if there are people hiding," he said.

Senator Bill Nelson told Fox News there had been "many deaths" at the high school.

Helicopter vision showed police loading a handcuffed person wearing a maroon jumper into the back of a police vehicle.

The shooting began just before school was due to finish for the day, after someone pulled the fire alarm, according to the Miami Herald. Students and teachers were puzzled because the school had already held a fire drill that day.

Then a code red, school lingo for a lockdown, was read over the loud speaker.

"Six kids ran back into my room, and I locked the door, turned out the lights and had the kids go to the back of the room," maths teacher Jim Gard said. "I told the kids to hang in there, it may still be a drill."

The Governor of Florida Rick Scott tweeted he has been in touch with local law enforcement, and said he was travelling to Broward County to be briefed on the shooting and meet with local law enforcement.

President Trump also tweeted, saying "no child, teacher or anyone else should ever feel unsafe in an American school". He said he spoke to Governor Scott and "we are working closely with law enforcement on the terrible Florida school shooting".

One student told Fox News said he was in his classroom when they heard shots.

"Everyone started freaking out," the student said.

"Everyone just started running towards the canal."

Another student said she heard a shot, but was near the exit and all her friends were able to evacuate.

Marjory Stoneman Douglas High School teacher Melissa Falkowski told CNN it had been "difficult" to put on a brave face for her students during the shooting.

"It was horrible," she said. "[We were] locked in the closet for 40 minutes."

Ms Falkowski said teachers had training for shooting incidents just six weeks ago, which meant they knew how to deal with the situation.

"It could have been a lot worse."

Parkland, an affluent Fort Lauderdale suburb with a population of about 30,000, is known for its good public schools. Douglas High is among the largest in the Broward school district, with about 3,200 students in grades 9 to 12.

The Marjory Stoneman Douglas High School shooting is the 18th school shooting in the US this year. Nine of those shootings involved injuries or fatalities, according to the Everytown log of school shootings.

In total there have been 290 school shootings in the US since 2013.

With wires.