US President Barack Obama has described new steps his administration will take to tighten gun rules and urged Americans to vote for candidates who do more to prevent gun violence, wiping back tears as he remembered children who died in a mass shooting.

Mr Obama delivered a powerful address in the White House, surrounded by family members of people killed in shootings.

His voice rose to a yell as he said the constitutional rights of Americans to bear arms needed to be balanced by the right to worship, gather peacefully and live their lives.

Mr Obama has often said his toughest time in office was grappling with the December 2012 massacre of 20 children and six adults at an elementary school in Newtown, Connecticut.

"Every time I think about those kids, it gets me mad," Mr Obama said, tears rolling down his cheek.

"That changed me, that day," he said, after being introduced by Mark Barden, whose seven-year-old son was killed in the shooting.

"My hope earnestly has been that it would change the country."

After that tragedy, the Democratic president failed to persuade Congress to toughen US gun laws.

He has blamed politicians for being in the thrall of the powerful National Rifle Association gun lobby group.

He said Americans need to be "just as passionate" as the NRA in changing gun laws, but acknowledged it will not happen during his final year in office.

Mr Obama laid out executive action he is taking to require more gun sellers to get licenses and more gun buyers to undergo background checks.

Under the changes, the US Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) will issue guidelines intended to narrow exceptions to a system that requires sellers to check with the FBI to determine whether buyers have criminal records, are charged with crimes or have mental health conditions that would bar them from owning a gun.

Any changes to gun rules in the US are fraught with political risk. The US Constitution's 2nd Amendment gives Americans the right to have arms, a right that is fiercely defended.

Legal challenges to the changes are expected and Republican presidential candidates have promised to reverse his order if they win the White House.

But Mr Obama, entering his eighth and last year in office, has said his measures are within his authority and consistent with the Constitution.

The NRA said the measures are "ripe for abuse" by the Obama administration and it will continue to fight to protect the constitutional right to keep and bear arms.

The lobby group said Mr Obama's proposals would not have prevented "any of the horrific" gun deaths he referenced in his remarks.

Mr Obama's critics described his gun control steps as an unlawful assault on Americans' constitutional rights, with Republican White House hopefuls pledging to immediately repeal the orders if they are elected in November.

White House candidate Jeb Bush warned that Mr Obama was "trying to do an end-run" on the US Constitution despite an increased terrorism threat.

"No matter what President Obama says, his word does not trump the Second Amendment," House Speaker Paul Ryan said in a tweet.

Several Democrats spoke out in support of Mr Obama's plans, including the three candidates running for their party's presidential nomination.

Frontrunner Hillary Clinton took to Twitter to thank Mr Obama "for taking a crucial step forward on gun violence. Our next president has to build on that progress - not rip it away."