An defiant and angry President Barack Obama Wednesday said the defeat of gun reforms in the US Senate was "shameful," accused senators of caving to the gun lobby, and promised to fight on.

A deeply emotional Obama -- in unusually direct language, and surrounded by relatives of gun victims including some of the 20 kids gunned down in the Newtown massacre -- accused the firearms lobby of lying to doom reforms.

He spoke soon after a bid to expand background checks for gun buyers, the most significant effort to change gun laws since the school killing last year, was blocked in the Senate by Republicans and some conservative Democrats.

"Families that know unspeakable grief summoned the courage to petition their elected leaders not just to honor the memory of their children but to protect the lives of all of our children," Obama said.

"A few minutes ago a minority in the United States Senate decided it wasn't worth it," the president said in the White House Rose Garden, as he absorbed the first significant political defeat of his second term.

"Instead of supporting this compromise, the gun lobby and its allies willfully lied about the bill," said Obama, who was clearly furious about the vote, months after vowing to use all his power to enact gun reform.

In a scathing rebuke, Obama said that Republicans and some Democrats had fallen prey to politics and had simply been scared that the wealthy gun lobby would come after them in future elections.

"They caved to the pressure. And they started looking for an excuse, any excuse to vote no."

The president however vowed to carry on fighting for reform.

"I see this as just round one," he said.

"I'm assuming our expressions of grief and our commitment to do something different to prevent these things from happening are not empty words.

"I believe we're going to be able to get this done, sooner or later we're going to get this right. The memories of these children demand it."