US Vice President Joe Biden warned Tuesday the world would be mystified if Republicans followed through on a "mind boggling" threat to block votes on new gun laws after the Newtown massacre.

Biden, who served in the Senate for nearly four decades, expressed disbelief that some former colleagues would block votes on White House-backed bills to curb gun violence after the killings of 20 kids and six adults in December.

"It appears that now not only are some of the senators not willing to stand and be counted, they're prepared to stop anybody from being able to be counted. I mean, it's almost mind-boggling," Biden said.

"What an embarrassing thing to say. Imagine what they are saying in other capitals around the world today. The rest of the world follows everything we do," Biden told law enforcement officers at a White House event.

"Imagine how this makes us look. I can't believe the Senate will actually do it," Biden said, in the latest effort by the White House to revive hopes of meaningful reform as prospects for the legislation hang by a thread in Congress.

"They're saying we're not even going to talk about this, a tragedy that traumatized the nation and caught the attention of the entire world."

Biden said he spent two hours Tuesday with family members of those killed at Sandy Hook Elementary School in Connecticut, a day after President Barack Obama traveled to the state to make an impassioned call for reform.

He told one heart-rending tale of a mother who lost a daughter in the attack who could not understand how politicians would not act on more gun control.

"One remarkable woman said... 'how do they explain not doing anything? My little girl, my baby was hiding in a bathroom, and she got shot through the heart'," Biden said.

"It is time for these guys to stand up and be counted... what are you going to say to those parents? Look them in the eye and tell them you concluded there is nothing you can do."

White House calls for a new ban on assault rifles and limits on the size of fast-firing magazines look unlikely to pass Congress amid opposition from the gun lobby, most Republicans and some Democrats from conservative states.

The best hope for reform now lies in a drive to expand background checks for all gun purchases -- but even that measure appears in doubt.

A 13-strong group of Republicans, including Senate minority leader Mitch McConnell, have threatened to use filibuster tactics to prevent gun bills even coming to a vote, saying they would infringe on the constitutional right to bear arms.