WASHINGTON — US lawmakers bolstered efforts Thursday to ban devices used by the Las Vegas shooter to make his guns fire faster, while the National Rifle Association unexpectedly urged federal officials to review the legality of such modifications.

The influential pro-gun lobby group broke from its traditional outright opposition to any gun control efforts by calling on the Bureau of Alcohol Tobacco, Firearms and Explosives (ATF) to consider changing the laws surrounding so-called "bump stocks."

"The NRA believes that devices designed to allow semi-automatic rifles to function like fully-automatic rifles should be subject to additional regulations," the NRA said.

The statement is a notable concession by the group, which has vehemently opposed any efforts to tighten gun laws or limit gun owners’ options to modify their weapons, and it could open the door to a broader debate about bump stocks.

But should the ATF modify federal statute to make such devices illegal, the move would circumvent Congress.

As police search for more clues into what drove Stephen Paddock to murder 58 people and wound nearly 500 at a country music concert, President Donald Trump’s White House also announced it was "open" to further debate about the devices.

The spring-loaded mechanism uses a rifle’s recoil to repeatedly and rapidly pull the trigger, allowing the user to fire several hundred rounds per minute.

"Members of both parties and multiple organisations are planning to take a look at bump stocks," White House Press Secretary Sarah Huckabee Sanders told reporters.

"We welcome that and would like to be part of that conversation."

Chicago, Boston probed

As Congress appeared prepared to at least consider moving forward on the first gun limits in years, it emerged that Paddock may have scoped out other major US cities for possible attacks.

Chicago’s Blackstone hotel said a man by the same name had reserved a room there in August -- but never showed -- as hundreds of thousands of people were attending the outdoor concert festival Lollapalooza, including Malia Obama, daughter of the former president.

He had also conducted internet searches in Boston, reported the Boston Globe, raising the prospect that Paddock may have been plotting more attacks.

The NRA and White House announcements give cover to Republican lawmakers, many of whom receive NRA funding, to back current legislation that would ban the sale of bump stocks.

"Clearly this is something we need to look into," House Speaker Paul Ryan, the top Republican in Congress, told MSNBC.

House and Senate Democrats have introduced bills banning bump stocks and similar devices, like trigger cranks, that can accelerate the firing rate of a semi-automatic weapon to nearly that of a machine gun.

Senator Diane Feinstein, whose assault weapons ban was defeated in 2013, four months after the Newtown shooting where 20 elementary school children were shot dead, said she hoped now was the time Republicans could support her measure to curtail use of the devices.

"Mr. President, you know what the right thing to do is," said Senate Minority Leader Chuck Schumer, calling on Trump, who visited Las Vegas Wednesday and met with victims and first responders, to support a ban on bump stocks.

While Republicans like Senators Lindsey Graham and John Cornyn were open to hearings on bump stocks, not all Republicans were on board.

"I think this is about chipping away at the Second Amendment," said Senator John Kennedy, referring to the clause in the US Constitution which guarantees citizens the right to bear arms.

Authorities meanwhile were studying the relationship between Paddock, who had no criminal record, and his girlfriend Marilou Danley, whom FBI agents questioned for clues to what drove Paddock to commit the deadliest US shooting of modern times.

Escape planned?

In a statement read by her attorney Matthew Lombard, the 62-year-old Danley said she had no hint of what was to come.

"He never said anything to me or took any action that I was aware of that I understood in any way to be a warning that something horrible like this was going to happen," she added.

Authorities have been at a loss as to why the 64-year-old gambler and retired accountant hauled a vast arsenal of weapons to his hotel room and launched his assault. — AFP