WASHINGTON -- US President Donald Trump on Thursday signaled an openness to raising the minimum age to buy some kinds of guns and tightening background checks, as the Florida school shooting prompted a public outcry for action.

"I will be strongly pushing Comprehensive Background Checks with an emphasis on Mental Health. Raise age to 21 and end sale of Bump Stocks," Trump said in a series of tweets on Thursday.

The president, however, didn't offer details of how he would push for any policy changes in the world's most developed country long been plagued by gun violence, with more than 300 million guns scattered nationwide.

Trump's remarks came a day after he heard pleas for solutions from grieving survivors of the Florida school shooting and family members of victims.

A 19-year-old gunman killed 17 people last week by firing an AR-15 style assault rifle at a high school in the state of Florida, prompting student-driven street protests across the country to demand stricter gun laws.

Students from the Florida high school are also planning a "March for Our Lives" rally in Washington on March 24 to call attention to school safety and urge lawmakers to enact gun control.

Per US federal laws, people must be at least 21 years old to buy a handgun from a licensed dealer, but the limit is 18 for rifles including assault-type weapons such as the AR-15 used by the Florida shooter in the deadliest high school shooting in US history.

Trump said earlier this week that he has recommended that "bump stocks", devices that enable semi-automatic weapons to fire hundreds of rounds per minute, be banned.

A professional gambler killed 58 people and injured more than 500 with rifles fitted with bump stocks in Las Vegas in the US state of Nevada last October in the worst mass shooting in morden US history.

Trump also suggested that the possibility of arming teachers as he vowed strong background checks and mental health screenings for gun buyers during the televised White House meeting with those affected by school shootings on Wednesday.

In one of the tweets Thursday, Trump clarified that what he said was to look at the possibility of giving "concealed guns to gun adept teachers with military or special training experience."

He said that highly-trained, gun-adept educators, if armed, would act before police and first responders get to the shooting site, and they would also serve as a deterrent to would-be gunmen.

"Must be offensive, defense alone won't work," Trump added.

But Trump also threw his support behind the National Rifle Association (NRA), a powerful U.S. organization that advocates for gun rights.

"What many people don't understand, or don't want to understand is that Wayne, Chris and the folks who work so hard at the @NRA are Great People and Great American Patriots," Trump wrote, referring the executive vice president Wayne LaPierre and executive director Chris Cox.

Trump received financial support from the NRA during the 2016 presidential campaign, but some of the gun control measures that appeared to have had his endorsement, including raising the minimum age to 21 for more weapons, run against what the NRA stands for.

Speaking at the Conservative Political Action Conference (CPAC) on Thursday, LaPierre lashed out at the FBI for its failure to follow on a tip on the Florida school shooter, and liberal critics who think gun control is the answer.

"They don't care if their laws work or not," LaPierre told the event. "They just want to get more laws to get more control over people."

LaPierre also said that criminals should not have access to a gun, and someone who shows waning signs of mental disturbance should show up in background checks.

Also addressing conservatives at CPAC, NRA spokeswoman Dana Loesch said that "many media love mass shootings."

"I'm not saying that you love the tragedy," Loesch said. "But I am saying that you love the ratings." She also said that Democratic lawmakers are taking advantage of the shooting. "They're exploiting a tragedy for an agenda," Loesch said.

In a statement on Wednesday, the NRA said that it "will continue oppose gun control measures that only serve to punish law-abiding citizens."

Trump was expected to meet with state and local officials on school safety on Thursday.