US President Barack Obama backs a new bill to revive an assault weapons ban and other new gun laws, the White House said Tuesday, fleshing out his nascent response to a shocking school massacre.

Obama also called an ardently pro-gun senator who has shifted his position on firearms laws since Friday's carnage in Connecticut and has begun meeting top cabinet officials to consider his options, his spokesman Jay Carney said.

The killings of 26 people, including 20 children, at a Newtown elementary school traumatized Americans, and may have shifted the political debate on firearms in US society, after years of gun lobby ascendancy.

Carney said that Obama is "actively supportive" of an effort by Democratic Senator Dianne Feinstein to write a bill early next year to reintroduce a ban on assault weapons that expired in 2004.

Obama aides have said after previous gun crimes that the president supports reintroducing an ban on deadly arms like the one used by Newtown gunman Adam Lanza, but he has never put personal political muscle behind such a push.

Carney said Obama would also interested in any move to ban high-capacity clips -- magazines that hold dozens of rounds -- and close the so-called "gun show loophole" that allows unlicensed individuals to sell guns privately.

"He is heartened... by what we have all heard from some members of Congress who have been long-time opponents of gun control measures, common-sense gun control measures like the assault weapons ban and the like," Carney said.

Feinstein has said her bill would ban by name at least 100 military-style semi-automatic assault weapons, and would curb the transfer, importation and the possession of such arms.

"It's going to be strong, and it's going to be definitive," she said.

But once the outrage from the Newtown massacre fades, prospects for Feinstein's bill remain uncertain and every piece of legislation is subject to intense amendment and pressure from various lobby groups.

But the California senator said that the tragedy was so acute in Newtown, a "sea change" in gun politics was possible.

"This is so graphic in people's minds, the smallness and beauty of these children, is so graphic, the loss is so dramatic," she said.

The most well known gun lobby group, the National Rifle Association, spoke up about the school carnage for the first time Tuesday, saying it was "shocked" and pledging to hold a news conference on Friday.

"The NRA is prepared to offer meaningful contributions to help make sure this never happens again," it said.

On Monday, the White House said Obama did not yet have a "specific" agenda on gun control, but would begin a dialogue with Americans on a broad approach to the issue within "weeks."

But it said that Obama's pledge on Sunday that gun tragedies "must end" could only be realized in part by gun control.

To that end, he met Education Secretary Arne Duncan, Health Secretary Kathleen Sebelius and Attorney General Eric Holder on Monday to discuss a "comprehensive" effort, possibly including new efforts to stop the mentally ill from getting guns.

Obama also Tuesday called West Virginia Senator Joe Manchin, a pro-gun politician who has suggested he may back an assault weapons ban, following the trauma of Newtown.

Americans meanwhile deeply affected by gun violence gathered to pressure politicians in Washington for change, under the auspices of the Brady Campaign to Prevent Gun Violence.

Andrei Nikitchyuk, father of a boy who escaped from the school in Newtown, called for "immediate change" as he stood with campaigners who had lost loved ones in shootings including those at Virginia Tech and Aurora, Colorado.

"We all need to speak up, let's unite, stop this partisan division. It's not a partisan issue," he said.

America has suffered an epidemic of gun violence over the last three decades, including 62 mass shooting sprees since 1982, three of the deadliest in the second half of this year alone.

The vast majority of weapons used have been semi-automatic handguns or military-style assault weapons obtained legally by the killers.

The right to bear arms is enshrined in the Second Amendment to the US Constitution, and many political leaders are pro-gun for political and philosophical reasons, though there are signs some positions are softening.

"We need to accept the reality that we're not doing enough to protect our citizens," Senate Democratic Majority leader Harry Reid said Tuesday.

"We must engage in a thoughtful debate about how to change laws and a culture that allows violence not to continue to grow. Every idea should be on the table."

Top Republican senator Mitch McConnell, a key figure in a party that has yet to embrace calls for a tweaking of gun laws, spoke of the need to enforce restrictions already on the books, but did not rule out change categorically.