The US Department of Justice has submitted a regulation that would ban so-called bump stocks, a type of gun modification that increases the rate of fire for semi-automatic rifles.

The proposal, submitted on Saturday, comes after President Donald Trump announced last month that he was in favour of the ban following a mass shooting in Florida.

"President Trump is absolutely committed to ensuring the safety and security of every American and he has directed us to propose a regulation addressing bump stocks," Attorney General Jeff Sessions said in a statement.

"To that end, the Department of Justice has submitted to the Office of Management and Budget a notice of a proposed regulation to clarify that the National Firearms and Gun Control Act defines "machine gun" to include bump stock type devices," he added.

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Under US law, machine guns are officially banned. However, semi-automatic rifles like the AR-15 are legal because of their lower rate of fire.

Bump stocks use the recoil of the semi-automatic rifle to allow the weapon to fire at a higher rate, effectively turning a semi-automatic into a machine gun.

Saturday's proposal would classify bump stocks as part of a "machine gun", making it illegal to buy or sell them.

The proposal must still be approved by the Office of Management and Budget before it is published and subject to a commentary period. It does not, however, require approval from Congress, where it may have faced opposition due the pro-gun ownership lobby's power.

Las Vegas shooting

Bump stocks were heavily criticised last October after one of the deadliest mass shootings in US history.

The gunman, 64-year-old Stephen Paddock, used the modifications to increase his rate of fire, allowing him to kill 58 people at a concert in Las Vegas.

Although Trump announced in October he would be in favour of the ban, no proposal was ever introduced.

The discussion again gained momentum after a gunman opened fire on a Florida high school last month, killing 17 people.

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Bump stocks were not used during the Florida shooting, but calls, led by students, for stricter gun control have been heard nationwide.

After February's shooting, Trump said he supported a ban on bump stocks, as well as potentially arming teachers - a proposal that has been criticised by many students and teachers.

On Thursday, state legislators in Florida passed a bill that, among other things, banned bump stocks on a state level.

That state law also raised the minimum age for all gun purchases from 18 to 21.

The bill did not outright ban the sales of AR-15 style assault rifles, which are often used in mass shootings, including both the Florida and Las Vegas shootings. Gun control advocates have long called for such a ban, but so far have not had any success.

Just hours after Florida Governor Scott Pruitt signed the Florida legislation on Friday, the National Rifle Association (NRA) filed a federal lawsuit against the state over the portion of the law that raises the minimum age to buy a firearm.