At least 17 people have been killed in a shooting at a school in south Florida on Valentine's Day.

Police have arrested Nikolas Cruz, a 19-year-old former student who was expelled, over the shooting at the school in Parkland.

Officers say he was equipped with at least one rifle and that the attack began outside the building.

Sixteen people were also injured and taken to hospital, including two of the 17 who eventually died.

Twelve of the victims have been identified from the school, Marjory Stoneman Douglas High School, Sheriff Scott Israel said.

This is believed to be the 18th shooting in US schools this year.

Sheriff Israel said Cruz was arrested without incident after he was located off the school grounds.

He said several SWAT teams were deployed during the afternoon and cleared every building at the school complex to ensure there were no other threats.

The FBI were called in to a scene which he described as "horrific".

Two students at the school told Good Morning Britain that the incident had not come as a surprise, describing the suspected attacker as "not a good kid".

"Ever since middle school he has always been very violent - just not in the right state of mind," Alaina Cruz said.

"He used to break things around the school just for the fun of it. He was not a good kid."

Sheriff Israel said of him: "We already began to dissect his websites and the things on social media that he was on and some of the things that came to mind are very, very disturbing."

Another student, Chad Williams, 18, said: "He was crazy about guns.

"He was kind of an outcast. He didn't have many friends. He would do anything crazy for a laugh, but he was trouble."

Donald Trump tweeted his condolences following the attack, saying the White House was working "closely" with the authorities.

He also said he was in close contact with Florida Governor Rick Scott.

"My prayers and condolences to the families of the victims of the terrible Florida shooting," he tweeted.

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

At a press conference, Governor Scott said the attack was an act of "pure evil" but refused to discuss gun controls.

Sheriff Israel earlier added that the suspect had at least one AR-15 rifle and multiple magazines.

While some of the victims were found outside the building, most were inside, he added.

In the wake of the attack a number of celebrities called for tighter gun controls in the US, including Kim Kardashian and Julianne Moore.

Boogie Nights actress Moore tweeted: "The 18th school shooting in the US since January. It is heartbreaking. What will it take to change our nation's gun laws ???"

Kardashian West said: "We owe it to our children and our teachers to keep them safe while at school. Prayers won't do this: action will. Congress, please do your job and protect Americans from senseless gun violence."

Singer Nancy Sinatra called on the Republican Party to pass "sensible" gun laws immediately.

"Seventeen more of our young people dead, killed by a shooter with an automatic weapon he had no business having," she added.

Talk show host Ellen DeGeneres said: "No words, no actions, no laws are enough until we end this epidemic of school shootings in our country."