US President Joe Biden has signed the most sweeping gun violence bill in decades, a bipartisan compromise that seemed unimaginable until a recent series of mass shootings, including the massacre of 19 students and two teachers at a Texas elementary school. 

"Time is of the essence. Lives will be saved," he said in the Roosevelt Room of the White House. 

Citing the families of shooting victims he has met, the president said: "Their message to us was 'do something'. How many times did we hear that? 'Just do something. For God's sake, just do something.' Today we did." 

The House gave final approval, following Senate passage, and Biden acted just before leaving Washington for two summits in Europe. 

"Today we say 'more than enough'," Biden said. "It's time, when it seems impossible to get anything done in Washington, we are doing something consequential." 

The legislation will toughen background checks for the youngest gun buyers, keep firearms from more domestic violence offenders and help states put in place red flag laws that make it easier for authorities to take weapons from people adjudged to be dangerous. 

The president called it "a historic achievement". 

Most of its US$13 billion (NZ$20b) cost will help bolster mental health programmes and aid schools, which have been targeted in Newtown, Connecticut, and Parkland, Florida, and elsewhere in mass shootings. 

Biden said the compromise hammered out by a bipartisan group of senators from both parties "doesn't do everything I want" but "it does include actions I've long called for that are going to save lives". 

"I know there's much more work to do, and I'm never going to give up, but this is a monumental day," said the president, who was joined by his wife, Jill, a teacher, for the signing. 

After sitting to sign the bill, Biden sat reflectively for a moment, then murmured: "God willing, this is gonna save a lot of lives." 

US President Joe Biden signs into law S. 2938, the Bipartisan Safer Communities Act gun safety bill, in the Roosevelt Room of the White House in Washington. Photo / AP 

He also said they will host an event on July 11 for lawmakers and families affected by gun violence. The president spoke of families "who lost their souls to an epidemic of gun violence". 

"They lost their child, their husband, and their wife. Nothing is going to fill that void in their hearts. But they led the way so other families will not have the experience and the pain and trauma that they had to live through." 

Biden signed the measure two days after the Supreme Court's ruling on Thursday (Friday NZ time) striking down a New York law that restricted people's ability to carry concealed weapons. 

And Saturday's ceremony came less than 24 hours after the Supreme Court overturned the Roe v Wade decision, which had legalised abortion nationwide for nearly five decades. 

"Yesterday, I spoke about the Supreme Court's shocking decision striking down Roe v Wade," Biden said. "Jill and I know how painful and devastating the decision is for so many Americans. I mean, so many Americans." 

He noted the abortion ruling leaves enforcement up to the states, some of which have already moved to ban abortion or will soon do so. Biden said his administration will "focus on how they administer it and whether or not they violate other laws, like deciding to not allow people to cross state lines to get health services". 

Asked by reporters about whether the Supreme Court was broken, Biden said: "I think the Supreme Court has made some terrible decisions." 

He walked away without answering more questions, noting: "I have a helicopter waiting for me to take off." 

While the new gun law does not include tougher restrictions long championed by Democrats, such as a ban on assault-style weapons and background checks for all firearm transactions, it is the most impactful gun violence measure produced by Congress since the enactment of a long-expired assault weapons ban in 1993. 

Enough congressional Republicans joined Democrats in supporting the steps after recent rampages in Buffalo, New York and Uvalde, Texas. It took weeks of closed-door talks but senators emerged with a compromise. 

Biden signed the bill just before departing Washington for a summit of the Group of Seven leading economic powers – the United States, Britain, Canada, France, Germany, Italy and Japan – in Germany. He will travel later to Spain for a Nato meeting. 

- by Will Weissert, AP