WASHINGTON (AFP) - United States (US) Senator Dianne Feinstein on Thursday introduced a toughened version of her assault weapons ban that expired in 2004, but she and other Democrats acknowledged an "uphill" battle to get it through Congress.

The new legislation, coming just six weeks after a mass shooting at an elementary school in Newtown, Connecticut, left 20 children and six adults dead, aims to ban military-style semi-automatic weapons, as well as magazines and other feeding devices that accept more than 10 rounds.

It would ban 157 specific firearms outright, including the AR-15 Bushmaster used in Newtown, while respecting hunters and sportsmen by excluding 2,258 models of "legitimate" hunting and sporting rifles.

"We have had enough," Ms Feinstein said at an event with several members of Congress - all Democrats - and law enforcement officials, as she stood before a board with 10 semi-automatic weapons clipped to it as a visual prop.

"These weapons do not belong on the streets of our towns, our cities, in our schools, in our malls, in our workplaces, in our movie theaters." Ms Feinstein's is the first major gun legislation introduced since President Barack Obama proposed a series of measures last week, including a renewed assault weapons ban, aimed at reducing gun violence.

The bill would also require background checks on the sale or transfer of all "grandfathered" assault weapons, a measure that works toward Mr Obama's bid to close background check loopholes for gun purchases.

The US gun lobby, led by the powerful National Rifle Association (NRA), is staunchly opposed to any legislation that would restrict gun rights.

Lawmakers eager to impose stronger gun control face a fight among Republicans in Congress, many of whom rejected Mr Obama's proposals out of hand and who say they see any tightening of gun laws as an infringement of the right to bear arms as enshrined in the 2nd Amendment to the US Constitution.

Ms Feinstein insisted her bill would not take weapons away from law-abiding gun owners.

"The purpose is to dry up the supply of these weapons over time," she said.

Importantly, it would also reduce the number of military features on an acceptable civilian rifle - such as a pistol grip, folding or detachable stock, or barrel shroud - from two to one.

"It is an updated, smarter and more robust version of the assault weapons ban" of 1994," said Senator Chuck Schumer, who worked on the original ban as a congressman.

Still, it remains to be seen if the contentious legislation will make it through today's bitterly divided Senate, as well as the Republican-held House of Representatives.

"This is really an uphill road," Ms Feinstein said.

"If anyone asks today 'Can you win this,' the answer is we don't know."

Senate Majority Leader Harry Reid, seen as pro-gun rights by the NRA, has suggested it would be unlikely for such a ban to make it to the floor for a vote.

But on Tuesday he said he was willing to "have a free amendment process" on gun legislation.

The Senate Judiciary Committee opens hearings next week on Mr Obama's proposals, and Mr Reid said he expects the committee to craft a bill.

"It may not be everything everyone wants. But I hope it has stuff that is really important," Mr Reid told reporters.

Senator Richard Blumenthal of Connecticut sounded hopeful that some Republicans could back the measure - or parts of it.

"Not all of it may pass at once but it has to be taken in steps, so votes will be taken on particular measures, and I think some will pass," he told reporters after speaking at Feinstein's event.

"A number (of Republicans) have said privately they are open to supporting reasonable measures for the first time ever." Vice-President Joe Biden, speaking in a Google+ "hangout" session to discuss Mr Obama's proposals, backed Ms Feinstein's measure but said he was "much less concerned" with assault weapons than with high-capacity magazines, or the need to impose tighter controls on who is allowed to possess a gun.

"It's not about keeping bad guns out of the hands of good people, it's about keeping all guns out of the hands of bad people," Mr Biden said.

"There should be rational limits."