U.S. President Barack Obama on Thursday mourned the victims of the Newtown shooting massacre over 100 days after the tragedy and urged Congress to move on gun- control legislation.

When the tragedy happened, the entire country was shocked and pledged to do something about it, said Obama, "Shame on us if we' ve forgotten."

Surrounded by weeping mothers of the children lost to gun violence, Obama made his emotional plea at a White House event and tried to build momentum for a package of gun-control legislation to be considered by the Senate in April.

"Tears aren't enough. Expressions of sympathy aren't enough. Speeches aren't enough. We've cried enough," said the president, stressing that it's the time to "turn heartbreak into something real."

The president went on to urge the passage of gun control measures, a universal background checks on gun purchases in particular.

"Now, in the coming weeks, members of Congress will vote on whether we should require universal background checks for anyone who wants to buy a gun so that criminals or people with severe mental illnesses can't get their hands on one," said Obama.

He said polling showed that 90 percent of Americans support the tightening of background checks.

"And this is our best chance in more than a decade to take common-sense steps that will save lives," he said.

The audience at the White House event included families of three of the 20 children who were killed in the Sandy Hook Elementary school mass shooting in Newtown, Connecticut last December. Also present was the mother of a Chicago girl who was killed on her way to school just days after performing at the president's second-term inauguration celebration in January.

The White House said earlier in the week that the president would advocate for his gun-control proposals in a series of public events in coming weeks.

Democrats in the Senate are expected to finalize a package of gun reform legislation to be introduced in the Senate. Senate Majority Leader Harry Reid said last week he would not include a renewal of the federal assault weapons ban in the package, which signaled the measure still lacked enough support in Congress.

Measures are expected to include providing more school safety aid, expanding federal background checks on gun sales and strengthening prosecution of illegal gun traffickers.

The Newtown tragedy has renewed gun control debate in the United States. Obama announced in January a comprehensive gun- control package of administrative actions while directing Congress to take up legislation on an assault weapons ban, a magazine size limit and a universal background check requirement.