President Barack Obama, with tears streaking down his cheeks, launched a final push to tighten sales of firearms, using his presidential powers in the absence of tougher gun restrictions that Congress refuses to pass.

The president struck a combative tone as he came out with plans for expanded background checks and other modest measures that have drawn criticism from gun rights groups, which Mr Obama accused of making Congress their hostage.

Palpable, too, was Mr Obama's extreme frustration at having made such little progress on gun control since the killing of 20 school children in Connecticut shocked the nation more than three years ago.

"First-graders," he said, resting his chin on his hand and wiping away tears as he recalled the 2012 massacre at Sandy Hook Elementary School in Newtown, Connecticut.

"Every time I think about those kids, it gets me mad."

Mr Obama's 10-point plan to keep firearms from those who should not have them marked a concession by the president, as he will leave office without securing the tough control laws he has repeatedly implored Congress to pass.

Although the president, acting alone, can take action around the margins, only Congress can enact more sweeping changes that gun control advocates say are the only way to truly stem the frequency of mass shootings.

"It won't happen overnight," Mr Obama said. "It won't happen during this Congress. It won't happen during my presidency."

But, he added optimistically: "A lot of things don't happen overnight."

The National Rifle Association, the largest gun rights lobbying group, slammed Mr Obama's plan and said it was "ripe for abuse," although the group did not specify what steps, if any, it will take to oppose or try to block it.

The centrepiece of Mr Obama's plan is an attempt to narrow the loophole that exempts gun sales from background checks if the seller is not a federal registered dealer.

With new federal "guidance," the administration is clarifying that even those who sell just a few weapons at gun shows, flea markets or online can be deemed dealers and required to conduct checks on prospective buyers.

Whether that step can make a significant dent in unregulated gun sales is an open question, and one not easily answered.

Millions of guns are sold annually in informal settings outside of gun shops, including many through private sales arranged online.

But the Obama administration acknowledged it could not quantify how many gun sales would be newly subjected to background checks, nor how many currently unregistered gun sellers would have to obtain a licence.

Easily reversible by a future president, the government's guidance to gun sellers lacks the impact of a new law, such as the one Mr Obama tried but failed to pass in 2013 in the wake of the Sandy Hook massacre.

Hoping to give the issue a human face, the White House assembled a cross-section of Americans affected by recent gun tragedies.

Mark Barden, whose son was shot dead at Sandy Hook, introduced the president with a declaration that "we are better than this".

Mr Obama readily conceded the executive steps will be challenged in court, a prediction quickly echoed by Republicans.

The White House expressed confidence the president was acting legally, and said Justice Department and White House lawyers had worked diligently to ensure the steps were watertight.

The announcement received predictably different reactions from presidential candidates.

Hillary Clinton and Bernie Sanders, both competing for the nomination from Mr Obama's Democratic party, pledged to build on his actions if elected.

The Republican field formed a chorus of voices vowing to annul the whole package, with Marco Rubio claiming: "Obama is obsessed with undermining the Second Amendment".

That amendment to the US Constitution guarantees the right to bear arms.

"Rather than focus on criminals and terrorists, he goes after the most law-abiding of citizens," said House Speaker Paul Ryan, a Republican.

"His words and actions amount to a form of intimidation that undermines liberty."