President Donald Trump and America's powerful gun lobby on Thursday cast citizens with weapons as a solution to shootings, as it emerged an armed deputy was on campus during a deadly Florida rampage but failed to act.

National Rifle Association chief Wayne LaPierre hit back at what he called "the shameful politicisation of tragedy" and repeated the organisation's position that "to stop a bad guy with a gun, it takes a good guy with a gun," while Trump made a controversial call to arm teachers.

The Broward County sheriff said Thursday that an armed deputy was in fact present during the Valentine's Day rampage that left 17 dead in a Florida high school, but did not act to stop it.

In his first public comments since the shooting, LaPierre reiterated long-standing accusations that gun control advocates were seeking to roll back the constitutional right to bear arms.

"It's a classic strategy right out of the playbook of a poisonous movement," he told an annual conservative conference outside Washington, hitting out at what he called "socialists" on the political left, and at the "so-called national news media."

"For them, it's not a safety issue, it's a political issue," he charged. "They hate the NRA. They hate the Second Amendment. They hate individual freedom."

Sheriff Scott Israel later announced that Scott Peterson, an armed school resource deputy, was at the high school but took up a position outside and "never went in" during the shooting.

He should have "went in, addressed the killer, killed the killer," Israel said.

Peterson resigned after being suspended without pay.

Two other deputies were placed on restricted duty during an investigation to determine if "they could have done more or should have done more" ahead of the shooting, Israel said.

Teachers warn of 'arms race' in schools

The NRA's cause received a significant boost when Trump -- in his second meeting at the White House on school safety in as many days -- floated a plan to respond to the Parkland carnage by putting more guns in schools.

He declared "gun free" schools a "magnet" for mass shooters and proposed bonuses for teachers who are willing to carry concealed firearms.

Trump had earlier proposed raising from 18 to 21 the minimum age to buy more guns than at present ─ like the assault-style rifle used by 19-year-old Florida shooter Nikolas Cruz ─ and making it more difficult for the mentally ill to own firearms.

Currently under federal law, anyone 18 or over can buy a gun from a private, unlicensed seller, although a handful of states have set the minimum age at 21.

Those measures, which may struggle to pass the Republican-controlled Congress, could have put him at odds with the NRA, which donated to and endorsed his campaign.

"I really think the NRA wants to do what's right," Trump said. "I mean, they're very close to me, I'm very close to them, they're very, very great people. They love this country. They're patriots."

Trump insisted he was not advocating arming every American teacher, but only those with "military or special training" ─ suggesting that would be around 20 per cent.

That would mean weapons for around 700,000 educators, a potentially massive business opportunity for gun manufacturers.

"A gun-free zone to a killer or somebody who wants to be a killer, that's like going in for the ice cream," he said. "That's like, 'Here I am, take me.'"

Teachers' unions were quick to condemn his proposal, with the American Federation of Teachers claiming Trump was in favour of an "arms race" that would "turn schools into militarised fortresses."

'Toxic lunacy'

"Anyone who wants guns in schools has no understanding of what goes on inside them — or worse, doesn't care," the union's president Randi Weingarten said.

Summing up the opposition of many lawmakers, Democratic Senator Richard Blumenthal called the idea of arming teachers "toxic lunacy ─ an NRA-backed distraction from common sense action.

"Arming teachers is inane and insane ─ a sure path to reckless and panicky shooting, gun theft, and other deadly dangers. A non-starter in the Senate."

The US Congress has long been deadlocked on the gun debate, accomplishing nothing despite a spate of mass shootings and polls showing that Americans support stricter gun laws by a two-to-one margin.

According to a Gallup tracking poll, 60 per cent of Americans now favour tougher gun sale laws.