Senate Democrats claimed a small victory early Thursday, forcing the US upper house to consider legislation to help keep guns out of the hands of terrorism suspects.

The move, which could break a yearslong logjam on gun control, follows demands for action after the weekend massacre of 49 people at a gay nightclub in Orlando, Florida.

Democrats took to the floor of the Republican-controlled Senate on Wednesday to launch a obstruction, known as a filibuster, to press colleagues to accept so-called "no-fly no buy" legislation.

Under it, people on watch lists or no-fly lists would be barred from buying firearms.

The filibuster was led by Senator Chris Murphy of Connecticut, where a 2012 school shooting left 20 children dead.

Murphy, with support from 40 senators, spent hours discussing ways to reduce gun violence. At 1:53 am on Thursday he claimed victory on Twitter.

"I am proud to announce that after 14-plus hours on the floor, we will have a vote on closing the terror gap and universal background checks," Murphy wrote.

The debate carried on as President Barack Obama prepared to visit Orlando to offer solace and healing to a distraught city.

Trump aims to change

In Orlando, Obama will confront an event whose causes seem to grow murkier by the day. Even as the families of the victims prepare to bury their loved ones, it's still unclear what confluence of factors led Omar Mateen, 29, to open fire in a gay nightclub where he may have been a frequent patron.

Investigators questioned Noor Salman on Wednesday, Mateen's wife, and a law enforcement source said she could face criminal charges if there is evidence of any wrongdoing on her part.

Presumptive Republican presidential nominee Donald Trump said on Wednesday he would meet with the National Rifle Association to ask the group to change its stance on allowing individuals on terrorist watch lists to buy guns. The NRA maintains such laws are ineffective.