US President Donald Trump has called to fast track the death penalty for "mentally ill monsters" who commit hate crimes and mass shootings but says guns aren't to blame.

In a White House address to the nation following two massacres, Mr Trump announced he was "directing the Department of Justice to propose legislation ensuring that those who commit hate crimes and mass murders face the death penalty", reports news.com.au.

He said capital punishment in the these cases of "barbaric slaughters" should be "delivered quickly, decisively, and without years of needless delay".

His comments come in the wake of the country's latest mass shootings, in El Paso, Texas, and Dayton, Ohio, which left 30 people dead and dozens injured over the weekend.

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The Justice Department is "seriously considering" bringing federal hate crime and federal firearm charges, which carry a possible death penalty, US lawyer John Bash said Sunday in a news conference.

Mr Trump called the gunman who killed 21 people in El Paso Texas a "wicked man" and condemned white supremacy.

"The shooter in El Paso posted a manifesto online consumed by racist hate. In one voice, our nation must condemn racism, bigotry and white supremacy," he said.

He told the nation he had directed the FBI to examine steps to identify and address domestic terrorism.

"These sinister ideologies must be defeated. Hate has no place in America," he said.

Accused El Paso shooter Patrick Crusius is believed to be the source of a four-page manifesto, which railed against immigrants, Hispanics and first-generation Americans, attached to a post on the 8chan bulletin board just minutes before the attack unfolded in a Walmart on Saturday.

The document also outlined the suspect's political and economic "justifications" for mass murder.

Patrick Crusius is believed to have authored the manifesto which was published online. Photo / Supplied
Patrick Crusius is believed to have authored the manifesto which was published online. Photo / Supplied

Just 13 hours later, nine more people were killed outside a bar in a popular night-life district in Dayton, Ohio.

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Mr Trump today described the "evil attacks" as "domestic terrorism" and "crimes against humanity". But guns are not the problem, according to the president.

He blamed video games and mental illness for the mass shootings but signaled he would oppose large-scale gun control efforts pushed by Democrats. He made no mention of more limits on the sales of actual firearms.

"Mental illness and hatred pulls the trigger, not the gun," Mr Trump said.

"We must recognise that the internet has provided a dangerous Ave to radicalised disturbed minds and perform demented acts.

"We must shine a light on the dark recesses of the internet and stop mass murderers before they start.

"The internet, likewise, is used for human trafficking, illegal drug distribution and so many other heinous crimes. The perils of the internet and social media cannot be ignored and they will not be ignored."

He urged Democrats and Republicans to set aside partisanship and find solutions to gun violence but offered few details on possible action. He told the US he wants legislation providing "strong background checks" for gun owners, though he has reneged on previous promises after mass attacks. In early 2018, he proposed increasing the minimum age required to buy assault weapons in the wake of the Las Vegas massacre, but backflipped after meeting with National Rifle Association leaders.

Mr Trump today called for a reduction in the "glorification" of violence in American culture, laws to make it easier to commit those with mental illness and "red flag laws" to separate such individuals from firearms.

"The choice is ours and ours alone, he said.

"It is not up to mentally ill monsters; it is up to us that we are able to pass great legislation. "After all of these years, we will ensure that those who were attacked will not have died in vain."

The president has come under fire for mistakenly referring to the town of Dayton as Toledo, during his Monday morning address. The cities are about 215kms apart.

"If we are able to pass great legislation after all of these years, we will ensure that those who were attacked will not have died in vain," he said.

"May God bless the memory of those who died in Toledo.

"May God protect them."

Leading Democrats were quick to criticise Trump for blaming mental illness and not raising the issue of gun control.