Live television footage showed students being evacuated, but the shooter remains at large, police said. Photo: ENV

The Broward Sherriff's Office said it was "working a developing incident" at Stoneman Douglas High School.

"There are reports of victims," it added, warning that the shooter was still at large and urging people to avoid the area.

US media outlets reported that up to 20 people may have been injured, while some have been evacuated.

A local Fox news affiliate reported seeing several people being treated for injuries outside the school.

The local public school district tweeted: "Today, close to Marjory Stoneman Douglas High School's dismissal, students and staff heard what sounded like gunfire."

"The school immediately went on lockdown but is now dismissing students. We are receiving reports of possible multiple injuries," it said.

Live television footage showed students being evacuated, but the shooter remains at large, police said

Video footage from the scene showed students being evacuated in small groups, while armed police patrolled the school's perimeter.

Coral Springs Police department, meanwhile, tweeted instructions to teachers and students to "remain barricaded inside until police reach you".

Police are working an active shooter scene, and ask that if you are in touch with your student you ask that they remain calm and barricaded until police come to their room, this is for everyone's safety. Do NOT call our 911 or non-emergency number unless it is an emergency.

The FBI also said it was attending the scene and assisting local law enforcement.

US television networks reported large numbers of parents had gathered outside the police perimeter.

President Donald Trump tweeted condolences "to the families of the victims". White House Deputy Press Secretary Lindsay Walters said the president had been briefed and that the White House was monitoring the situation.

Congressman Ted Deutch, the district's representative, tweeted: "I'm sick about this news from home. Just spoke with the sheriff. This is devastating."

He said it was an "incredible school in my district - great teachers and staff, involved parents and amazing kids. Spoke with the kids there just a few weeks back."