White House hopeful Hillary Clinton on Monday issued a pointed warning to US allies in the Middle East, saying countries like Saudi Arabia must crack down on citizens supporting extremism.

Following the weekend massacre at a gay club in Orlando by a gunman who the FBI says may have been radicalized, the presumptive Democratic presidential nominee vowed to step up efforts to stop so-called lone wolf attacks, and said Americans should unite to defeat domestic terrorism.

But she also called on other governments to reconsider policies that allow extremism to thrive, calling out three US allies which already have sensitive ties with Washington.

"It is long past time for the Saudis, the Qataris, and the Kuwaitis and others to stop their citizens from funding extremist organizations," Clinton said in a national security speech in Cleveland, Ohio.

"And they should stop supporting radical schools and mosques around the world that have set too many young people on a path toward extremism."

In her half-hour speech, the former secretary of state said "American leadership" was crucial in resolving political conflicts that fuel Islamic State extremists, as well as waging the immediate battle against terror groups and messages, including online. 

"The Orlando terrorist may be dead, but the virus that poisoned his mind remains very much alive and we must attack it with clear eyes, steady hands, unwavering determination and pride in our country and our values," she said. 

"The threat is metastasizing," she added.

"We face a twisted ideology and poisoned psychology that inspires the so-called lone wolves," whom she described as radicalized individuals who may or may not have direction from a formal organization.

US officials have said they do not believe the Orlando shooter, Omar Mateen, had outside direction from abroad in carrying out the attack that left 49 dead.

Clinton also renewed her call for an assault weapons ban, saying "weapons of war have no place on our streets." She mentioned in particular the AR-15, the style of semi-automatic rifle used in Orlando.

And she insisted that if federal authorities are tracking someone for suspected terror ties, that person "shouldn't be able to just go buy a gun with no questions asked."

"If you're too dangerous to get on a plane," she added, "you are too dangerous to buy a gun in America."

Source: AFP