WASHINGTON (AFP) - US Senate Democrats on Tuesday demanded a vote on a gun bill as Vice-President Joe Biden slammed a Republican threat of a filibuster as "mind-boggling" in the wake of the Newtown massacre.

Senate Majority Leader Harry Reid said he was shocked that 13 conservative senators had lined up to block the vote, and promised to overcome the tactic, known as a filibuster, with a procedural vote on Thursday.

"We need to move to this legislation," Mr Reid told reporters, weeks after the Judiciary Committee approved a three-part bill calling for background checks for all gun sales, felony penalties for gun trafficking, and new funding to boost school safety.

"It would be a real slap in the face to the American people not to do something on background checks, on school safety, on federal trafficking, which everybody thinks is a good idea," he said.

Mr Reid warned that inaction would show lawmakers' failure to keep America's children safe from gun violence.

"We have a responsibility to safeguard these little kids," Mr Reid said, citing the 20 children killed along with six adults in Newtown, Connecticut last December.

"We are really failing. We need to do more." This week's effort coincided with President Barack Obama's 11th hour appeal to Congress to allow a vote.

Mr Obama heaped on the pressure, with phone calls to Republican and Democratic senators on Tuesday to discuss gun violence prevention measures, according to a White House official.

He faces resistance from Republicans in conservative-leaning states where gun cultures like hunting are prevalent, and where efforts to crack down on gun violence are seen by many voters as infringing on the constitutional right to bear arms.

Democrats in some conservative states were keeping their options open.

Senator Max Baucus said he was not sure if he would vote to move to the gun legislation.

"My primary focus is the state of Montana. They're my employers," the Montana Democrat said.

Meanwhile in a concerted push by the White House, Mr Biden warned that the world would be mystified if Republicans followed through on the filibuster threat.

"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," Mr Biden said.

"What an embarrassing thing to say," he added. "Imagine what they are saying in other capitals around the world today." Mr Biden, who served in the Senate for nearly 40 years, sounded exasperated at the thought that "the climax of this (Newtown) tragedy could be we're not even going to get a vote."

The vice-president said he spent two hours on Tuesday with family members of those killed at Sandy Hook Elementary School in Connecticut, a day after Mr Obama travelled to the state to make an impassioned call for reform.

Many of those relatives are expected to be lobbying lawmakers this week.

"It is time for these guys to stand up and be counted," Mr Biden said of his former colleagues in the Senate.

"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 a handful of 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.

Democrats, meanwhile, are frantically seeking bipartisan backing for the legislation.

Democrat Joe Manchin is in negotiations with Pennsylvania Republican Pat Toomey over new, softer language in an amendment that would expand mandatory background checks to include sales at gun shows, which are currently exempt from such checks.

Top Senate Republican Mitch McConnell joined the filibuster effort this week, saying on Tuesday that the bill "clearly had no bipartisan support in committee." But at least 10 Senate Republicans have told reporters they would like to have an open debate and vote on gun legislation.

Mr Reid has said he would allow senators to offer various amendments to improve the bill, a gesture that led Republicans like Lindsey Graham to support allowing a vote.

"If your goal is to have a process that allows people to read it, understand it, debate it, amend it and vote on it, count me in," Mr Graham said.