Orlando nightclub massacre 176th mass shooting in past 168 days in US

"Those who were killed and injured here were gunned down by a single killer with a powerful assault weapon," Obama told reporters. "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."

"I truly hope that senators rise to the moment and do the right thing. We can stop some tragedies. We can save some lives. If we don't act, we will keep seeing more massacres like this," he added.

According to federal investigators, Mateen earlier this month legally purchased an assault rifle and a handgun, with which he appeared to launch his mass killing without external instruction.

Though assault-style weapons were banned in 1994, the US Congress refused to renew the ban when the prohibition expired in 2004. Gun-rights advocates argued that rifles of any type were scarcely used in homicides in the country.

Following the 2012 school shootings in Newtown, Connecticut, which claimed 28 lives, including 20 children, the Obama administration initiated but failed to push stronger gun control laws.

The laws, whose sections included expanded background checks and bans on assault weapons, were stymied in Congress after staunch opposition from Republican lawmakers and gun-rights lobby groups.

During his presidency, Obama has been confronted with more than a dozen of high-profile mass shootings, and in an interview last year he called the failure to reform US gun laws "one of the greatest frustrations" of his presidency.

"If you ask me where has been the one area where I feel that I've been most frustrated and most stymied, it is the fact that the United States of America is the one advanced nation on Earth in which we do not have sufficient common-sense gun safety laws, even in the face of repeated mass killings," Obama told BBC in July, 2015.