US President Barack Obama has called on lawmakers to take action to prevent gun violence after meeting with families of the victims of the Orlando nightclub shooting that killed dozens of Americans last weekend.

Omar Mateen, armed with assault weapons, stormed the Pulse Club on Sunday, killing 49 people and injuring 53 others at the gay club, marking the worst ever mass shooting in US history.

The 29-year-old suspect was an American-born US citizen born to parents of Afghan background. He was allegedly a Daesh sympathizer.

On Thursday, Obama and Vice President Joe Biden placed 49 white roses at a memorial for the 49 people who lost their lives in the shooting after they met with the families of the victims, police and hospital workers.

"Our hearts are broken, too. We stand with you," Obama said. "We are here for you."

“Today, once again, as has been true too many times before, I held and hugged grieving family members and parents and they asked, ‘why does this keep happening?’ ” Obama said, standing with Biden.

“They pleaded that we do more to stop the carnage,” he added. “They don’t care about the politics. Neither do I, neither does Joe, and neither should any parent out there. This debate needs to change.”

"Those who were killed and injured here were gunned down by a single killer with a powerful assault weapon," Obama said. "The motives of this killer may have been different than the mass killers in Aurora, or Newtown. But the instruments of death were so similar. Now another 49 innocent people are dead. Another 53 are injured. Some are still fighting for their lives."

Greatest frustration of Obama presidency

Obama has admitted that the greatest frustration of his presidency has been his failure to pass "common sense gun safety laws" in the United States.

Earlier this year, he made a passionate plea about the urgency to fight gun violence in the country, blasting Congress for inaction in the face of too many tragic deaths by firearms.

He announced a raft of executive actions to curb the gun violence, but added that Congress, which blocked a tougher gun bill in 2013, still needs to act to reform the US gun laws.

Obama slammed those members of Congress who have been taken “hostage” by the powerful gun lobby, and put their desire to get elected before the safety of American citizens.

Later on, Obama said America should learn a lesson from Australia, where firearms are strictly controlled and gun violence is almost unheard of.

The Republican-controlled Congress has snubbed efforts of the Obama administration to tighten gun laws.

Obama urges pro-gun people to meet victims

On Thursday, Obama said, “The notion that the answer to this tragedy would be to make sure that more people in a nightclub are similarly armed to the killer defies common sense.”

“Those who defend the easy accessibility to assault weapons should meet these families and explain why that makes sense,” he added.

Obama praised the US Senate for agreeing to vote on measures to prevent people on terror watch lists from buying firearms.

"I truly hope that senators rise to the moment and do the right thing,” he said. “We will not be able to stop every tragedy … but we can stop some tragedies. We can save some lives. We can reduce the impact of a terrorist attack if we’re smart.”