Republican White House hopeful Donald Trump signalled Wednesday he could support banning people on terror watch lists from purchasing guns, a move that would place him in opposition to members of his own party.

Trump, the presumptive Republican presidential nominee, regularly touts his support for the constitutionally enshrined right to bear arms.

He has said several times after terrorist attacks, including the mass shooting in Orlando, Florida Sunday, that death tolls would have been lower if private citizens had been armed and able to shoot back.

But he suggested he is prepared to consider restrictions on gun purchases, after it was revealed the Orlando shooter legally bought a rifle and handgun in Florida despite having been investigated and interviewed by the FBI for possible extremist ties.

"I will be meeting with the NRA, who has endorsed me, about not allowing people on the terrorist watch list, or the no fly list, to buy guns," Trump tweeted Wednesday.

Trump`s startling announcement may have placed him on a collision course with the National Rifle Association, one of the most powerful lobby groups in the nation.

On Tuesday the NRA tweeted: "Restrictions like bans on gun purchases by people on `watch lists` are ineffective, unconstitutional, or both."

According to Senate Democrat Bill Nelson of Florida, the Federal Bureau of Investigation said Omar Mateen, the gunman who killed 49 people at an Orlando gay nightclub, was placed on a terrorism watch list from 2013 until 2014.

Republican lawmakers and the National Rifle Association have refused to support legislation that would deny weapons to people on such lists, arguing that such a bill would infringe on the Second Amendment rights of everyday Americans, including those who may have been placed unfairly on watch lists or no-fly lists.

A Senate measure that would have prevented FBI terror suspects from purchasing firearms and explosives failed last December, with every Senate Republican but one voting in opposition.

President Barack Obama, speaking Tuesday on the Orlando tragedy, reiterated his backing for so-called "no-fly, no-buy" legislation.

"People with possible ties to terrorism who are not allowed on a plane shouldn`t be allowed to buy a gun," Obama said.

A US government report shows that known or suspected terrorists have passed background checks to purchase guns or explosives more than 90 percent of the time.

Democratic Senator Dianne Feinstein introduced a bill that would deny weapons to people on the watch lists, as well as to anybody the US attorney general suspects of terrorist ties.

Nelson filed legislation Wednesday that would require the FBI to be automatically notified of background checks for anyone who has been investigated for possible terrorism and attempts to buy a gun.