US President Donald Trump called the suspect in the mass school shooting in Parkland, Florida, "mentally disturbed" and vowed to help local jurisdictions tackle mental health issues, but he made no mention of stricter gun control laws.

In a televised address at the White House, Trump focused his response on the need for the nation to offer more support for young people who feel isolated a day after Nikolas Cruz, 19, a former student who had been expelled from Marjory Stoneman Douglas High, was accused of the rampage that killed 17 people at his former school.

"I want you to know you are never alone and never will be," Trump said during brief remarks in the Diplomatic Room. He urged young people to turn for help to "a teacher, a family member, a local police officer or a faith leader."

"Answer hate with love," he said. "Answer cruelty with kindness."

Advertisement

Trump pledged his administration would help "tackle the difficult issue of mental health" and said the issue of improving safety in schools would be the top priority during a meeting later this month with governors and state attorneys general. Yet Trump made no mention of gun control laws in the aftermath of the third deadliest school shooting in US history.

In a tweet earlier Thursday, Trump implored the public to report erratic behaviour to authorities as a way to head off such rampages. The president said there were "so many signs" about Cruz and he declared, "Neighbours and classmates knew he was a big problem."

The president attached - or "threaded" - his tweet to an unrelated one from two days earlier that dealt with the congressional negotiations on immigration. It was not clear why.

He tweeted: So many signs that the Florida shooter was mentally disturbed, even expelled from school for bad and erratic behavior. Neighbors and classmates knew he was a big problem. Must always report such instances to authorities, again and again!"

Trump didn't mention gun control when addressing the nation after Nikolas Cruz's deadly attack on his former high school. Photo / Broward County Jail
Trump didn't mention gun control when addressing the nation after Nikolas Cruz's deadly attack on his former high school. Photo / Broward County Jail

On Wednesday, he offered his "prayers and condolences" to the families of the victims. Seventeen people have been reported killed in the massacre.

The White House cancelled its daily news briefing on Wednesday in the wake of the shooting. Trump has no public events on his calendar, which includes a private intelligence briefing and a private meeting with UN Ambassador Nikki Haley.

Trump's suggestion that Cruz, who authorities said used an AR-15 semiautomatic rifle, was mentally ill comes amid continued public debate over gun control laws and the mental health care system. Congress has not passed major new laws on guns despite an increase in the numbers of people killed in some of the biggest mass shootings.

The Trump White House has not pushed for any new gun laws.

It's not clear what recourse, if any, authorities would have had to stop Cruz even had they been made aware of his erratic behaviour before the shooting. Authorities said his social media postings included pictures of firearms, but Cruz purportedly purchased his rifle legally.

In a presidential proclamation, Trump ordered all American flags at the White House, federal buildings and US embassies flown at half-staff to honor the shooting victims.