President Donald Trump File picture: AP Photo/Manuel Balce Ceneta.

White House - President Donald Trump declared the United States a country in mourning Thursday, as he announced he would visit the scene of a Florida high school shooting that left 17 dead.

"My fellow Americans, today I speak to a nation in grief," Trump said in a televised address following the 18th mass shooting to hit a US school since the start of the year. 

In his address, the US leader avoided all mention of firearms, or the politically fraught issue of gun control, vowing instead to "tackle the difficult issue of mental health."

Earlier Thursday, Trump issued a largely symbolic proclamation, ordering that flags be flown at half staff at US embassies, government buildings and military installations.

"Our nation grieves with those who have lost loved ones in the shooting at the Marjory Stoneman Douglas High School in Parkland, Florida," he said.

But Trump's muted response to the tragedy has been striking.

In similar situations, previous presidents have appeared before the nation to console or unite. 

Barack Obama's tearful appearance after 20 elementary school children were cut down at Sandy Hook in 2012, was a seminal moment of his presidency.

Trump, as ever in his ground-breaking and rule-breaking presidency, has chosen to do things differently. 

At exactly the time US authorities confirmed 17 people had been shot dead in a school in Florida Wednesday evening, the White House said Trump would not appear again that evening.

In an unusual scene, reporters who work in what could be the world's busiest newsroom walked out the door toward Valentine's Day appointments they had assumed they would miss.

- A pair of tweets -

Trump's only comments Wednesday were a pair of tweets saying he spoke to Florida Governor Rick Scott and was "working closely with law enforcement on the terrible Florida school shooting." 

"My prayers and condolences to the families of the victims of the terrible Florida shooting. No child, teacher or anyone else should ever feel unsafe in an American school." 

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First Lady, Melania Trump, sent a tweet and so did Vice President Mike Pence.

But with other questions brewing about Trump's position on gun control, his alleged affair with a porn star and a battery of allegations against a top aide, the White House cancelled its already delayed regular daily briefing.

"In light of this tragedy there will not be a regularly scheduled briefing as previously announced," Sanders said, three hours after a briefing had been due to start.

No briefing was scheduled for Thursday either. 

Inside any White House, there is a difficult conversation about what to say after a mass shooting and how to say it. 

"Somehow this has become routine," Barack Obama said in 2015. By the end of his presidency, he had, he said, felt tapped out, unsure what to say and how it could make a difference. 

"The reporting is routine. My response here at this podium ends up being routine." The response is difficult, in part, because Americans' differences over gun laws are so engrained.

This latest mass shooting -- in a country where mass shootings are an almost daily occurrence -- has inevitably reignited questions about America's permissive gun laws.

Trump's hard opposition to any additional gun control makes this White House's response even more difficult.

He is the first president to have addressed the NRA, a powerful gun industry group that has outsized influence over American politics.

Sales of weapons have actually declined since he was elected, with owners feeling less of a need to buy ahead of possible curbs.

- Onus on shooter -

By Thursday morning Trump again took to Twitter to put the onus on the alleged shooter, urging Americans to report neighbors' "erratic" behavior.

"So many signs that the Florida shooter was mentally disturbed, even expelled from school for bad and erratic behavior," Trump tweeted.

"Neighbors and classmates knew he was a big problem. Must always report such instances to authorities, again and again!"

The suspect used an Colt AR-15, a semi-automatic weapon that was reported to have been legally purchased.

Opponents of gun control have sought to shift the debate to focus on the behavior and motives of those using the weapons.