Orlando shooting self-radicalisation, not directed attack: US

America's deadliest ever shooting that killed 49 people and injured 53 others in Orlando was a case of self-radicalisation and was not a terrorist-directed attack, a top US official said today.

"What we know at this point, it appears that this was a case of self-radicalisation, it was not a terrorist-directed attack," Secretary of Homeland Security Jeh Johnson told Fox News in an interview.

"As we say, it was more in the nature of a terrorist- inspired attack. He does not appear to have been part of any group or any cell," Johnson said in response to a question on the alleged suspect Omar Mateen.

Afghan-origin Mateen, 29, investigators say was self-radicalised and pledged allegiance to Islamic State (IS).

"And the environment we're in right now is reflected by this horrific attack where we have to be concerned about the homegrown violent extremism," he said.

"For both the President and me, homeland security, the protection of the American people against this type of threat is our number one priority," Johnson said.

Johnson said there's some indication at this point that he self-radicalised based on things he was looking at on the Internet and on social media.

"(This) is why one of the things that we're focused on in homeland security is how do we encourage Internet service providers to take down prohibited content, content that violates the terms of their own service," he said.

"They're pretty good about doing this. But it becomes a more and more difficult task as the terrorists become smarter at this," he added.

Johnson said the US needs to be concerned about homegrown violent extremists.

"We are taking the fight to the Islamic State pursuant to the President's direction. We're killing these guys where they rear their heads in places like Iraq and Syria," he noted.

"But we've got to be concerned about those here in the United States who self-radicalise, which is why our whole of government efforts to take them down with law enforcement, to build bridges to the American Muslim community so that they help us help them has become all the more important," he said.

Earlier in the evening, President Barack Obama at a meet with Democratic lawmakers said this has been a difficult week for the country.

"Obviously this has been a difficult week for America, because all of us are still grieving for those who were lost in Orlando. All of us still have our thoughts and prayers for the families, those who were killed but also for those who are still recovering and for the city of Orlando," he said.

"One of the things I said I emphasised this is something that could happen anywhere. These could be our kids or our brothers or our cousins or our nephews, nieces," Obama said.

"And at moments like this it's critically important for us to remind ourselves of what binds us together as a people. That regardless of race or ethnicity or religion or sexual orientation, we're all Americans and we look out for each other. We celebrate those things we hold dear and have in common, like love of family and love of country, we mourn together when part of that family is hurt," he said.

Meanwhile, Senator Ron Johnson, chairman of the Senate Homeland Security and Governmental Affairs Committee, sent a letter to G4S Secure Solutions, the security company that employed Mateen seeking more information about him.

Cartoon by - Nirjhar Som