The Trump administration has formally proposed a complete ban on bump stocks, which effectively convert assault rifles into machine guns.

The Justice Department's proposed rule would order the surrender or destruction of more than a half-million of the devices estimated sold over the past decade.

Several bump stocks were used by the gunman who shot concert-goers in Las Vegas on 1 October, killing 58 people and injuring more than 850.

The proposed rule will include a 90-day period for people to comment on it - possibly leading to changes - and then it will become law.

"As I promised, today the Department of Justice will issue the rule banning BUMP STOCKS with a mandated comment period," President Donald trump tweeted.

"We will BAN all devices that turn legal weapons into illegal machine guns."

The move came a day before hundreds of thousands of people around the country were expected to join marches in support of greater gun controls in the aftermath of the Parkland school shooting in which 17 people lost their lives.

Students who survived the attack have organised the march, with massive crowds expected in Washington. 

Attorney General Jeff Sessions said the new rule would classify bump stocks under the national ban on machine guns. 

Several years ago, federal regulators said the devices, which are mechanical attachments to the stock of a rifle that make it fire rapidly, could not be classified that way.

And Congress, where the National Rifle Association pro-gun lobby has hefty influence, has been reticent to act.

"After the senseless attack in Las Vegas, this proposed rule is a critical step in our effort to reduce the threat of gun violence that is in keeping with the Constitution and the laws passed by Congress," Mr Sessions said.

In its proposal, the Justice Department estimated that as many as 520,000 of the devices have been sold around the country since they were first marketed from around 2008. 

There are two main manufacturers, who have continued to sell them despite the evidence of their danger after the Las Vegas massacre. 

Sessions said owners would "be required to surrender, destroy, or otherwise render the devices permanently inoperable" once the proposal becomes law.