Two mass shootings that killed 29 people in Texas and Ohio have reverberated across the United States, as Democratic presidential candidates called for stricter gun laws and some accused President Donald Trump of being a white nationalist.

Key points: In Dayton, Ohio, a gunman killed nine people, including his sister

In Dayton, Ohio, a gunman killed nine people, including his sister The El Paso shooting is being investigated as a possible hate crime

The El Paso shooting is being investigated as a possible hate crime The attacks come less than a week after a 19-year-old gunman killed three people

Dozens were also wounded on Saturday and early Sunday in the attacks, which occurred within just 13 hours of each other.

The carnage has shocked a country that has become grimly accustomed to mass shootings and heightened concerns about domestic terrorism.

The first massacre occurred on Saturday morning (local time) in the heavily Hispanic border city of El Paso, where a gunman killed 20 people at a Walmart store before surrendering.

Authorities have linked a racist, anti-immigrant screed posted online shortly before the incident to the man arrested over the shooting, 21-year-old Patrick Wood Crusius.

CCTV images of the El Paso gunman have emerged. ( AFP: photo courtesy of KTSM 9 )

In Ohio, a gunman opened fire in a downtown district of Dayton early on Sunday killing nine people, including his sister, and wounding at least 26 others.

The assailant was killed by police, adding to the death toll.

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Police have named the gunman as Connor Betts, a 24-year-old man of Caucasian descent.

The attacks came less than a week after a 19-year-old gunman killed three people and injured 13 others at the popular Gilroy Garlic Festival in California before dying of a self-inflicted gunshot wound.

The shootings were the 21st and 22nd mass killings of 2019 in the US, according to a mass murder database that tracks homicides where four or more people are killed — not including the offender.

According to the non-profit Gun Violence Archive, there have been 8,752 gun-related deaths in the US so far in 2019, not including suicides or accidents.

US President Donald Trump has ordered that all flags be flown at half-mast in remembrance of the victims of the weekend shootings.

He told reporters in New Jersey that "hate has no place in our country" and said he would present a full statement on Monday.

A proclamation released by the White House on Sunday said the nation shared "in the pain and suffering of all those who were injured in these two senseless attacks".

Donald Trump has ordered that all flags be flown at half mast in remembrance of the victims. ( AP: Andrew Harnik )

Democrats say Trump's rhetoric fuels capacity for violence

Federal authorities said the El Paso shooting would be handled as a domestic terrorism case and charges could carry the death penalty.

A local prosecutor announced that he would bring capital murder charges against the suspect, saying the assailant "lost the right to be among us".

The border city is home to 680,000 people, many of them Latino.

Mexican Foreign Minister Marcelo Ebrard said Mexico would take legal action to protect Mexicans and Americans of Mexican descent after the shooting in El Paso, Texas.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 37 seconds 37 s A man in handcuffs is led away by police near El Paso Walmart shooting. (Credit: Ayla Pequeño)

In a video statement, Mr Ebrard called the shooting an "act of barbarism" and said the country's first priority was helping the families of the victims.

Speaking in downtown El Paso, League of United Latin American Citizens president Domingo Garcia said that "unfortunately what we saw here was another massacre by again somebody using racial hatred as a basis to kill people of Mexican-American descent, and we need to stand up and fight against it."

He also called on Mr Trump and Texas Governor Greg Abbott to stop fuelling anti-immigrant rhetoric that was "costing the lives of innocent people".

It was a personal issue for Democratic presidential candidate Beto O'Rourke, the former congressman who returned to El Paso after the attack in his hometown.

Asked on CNN if he believed Mr Trump was a white nationalist, he responded, "Yes, I do".

Mr O'Rourke visited victims of the mass shootings in his hometown of El Paso. ( AP via Beto O'Rourke Facebook )

"Let's be very clear about what is causing this and who the President is," Mr O'Rourke said.

"He is an open avowed racist and is encouraging more racism in this country."

US Senator Bernie Sanders said he agreed that Mr Trump was a white nationalist, and said the President's rhetoric unduly influenced unstable people to take up arms.

"It gives me no pleasure to say this but I think all of the evidence out there suggests that we have a president who is a racist, who is a xenophobe who appeals, and is trying to appeal, to white nationalism," Mr Sanders told CNN.

Acting White House Chief of Staff Mick Mulvaney rebutted the Democrats' allegations and attributed the shootings to "sick" individuals.

"There's no benefit here in trying to make this a political issue. This is a social issue and we need to address it as that," he told American broadcaster ABC.

Alleged shooter spoke of 'Hispanic invasion'

Critics of Mr Trump have called the President a white nationalist. ( AP: Alex Brandon )

A hallmark of Mr Trump's presidency has been his determination to curb illegal immigration.

The President has drawn criticism for comments disparaging Mexican immigrants and referring to the flood of migrants trying to enter through the US southern border as an "invasion."

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In recent weeks, critics also accused Mr Trump of racism, after his attacks on members of Congress who belong to racial or ethnic minorities.

However, the President has repeatedly denied he is a racist.

The White House cannot shirk its responsibility in shaping public discourse, Democratic presidential hopeful Pete Buttigieg told Fox.

"There's no question that white nationalism is condoned at the highest levels of our government," he said.

"He's spoken about immigrants as being invaders. He's given licence for this toxic brew of white supremacy to fester more and more in this country, and we're seeing the results of that," Julian Castro, the former mayor of San Antonio, told ABC America.

While authorities were still investigating the motive of the El Paso shooter, Texas Governor Greg Abbott said the rampage appeared to be a hate crime.

The FBI has provided the image of the alleged El Paso shooter, Patrick Crusius. ( AP via FBI )

Police cited a manifesto they attributed to the suspect as evidence that the bloodshed was racially motivated.

The statement called the Walmart attack "a response to the Hispanic invasion of Texas."

The manifesto also expressed support for the gunman who killed 51 people at two mosques in Christchurch, New Zealand, in March.

The shooting renewed attention on domestic terrorism.

FBI director Christopher Wray told a Senate hearing in July the majority of the incidences of domestic terrorism the FBI had investigated were "motivated by some version of what you might call white supremacist violence."

Former vice-president Joe Biden launched his presidential campaign with a reminder of Mr Trump's response to the deadly 2017 attack at a white nationalist rally in Charlottesville, Virginia, when he said there were "fine people" on both sides.

Mr Biden refrained from attacks on Mr Trump on Sunday, instead calling for action to end "our gun violence epidemic".

Democrats call for emergency Congress session

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 34 seconds 34 s First responders to the Ohio shooting reached victims within a minute of the suspect opening fire.

The carnage ranked as the eighth-deadliest mass shooting in modern US history, after a 1984 shooting in San Ysidro, California, in which 21 people died.

According to the US Centres for Disease Control, in 2017 firearm homicides killed 14,542 people in the US.

Despite several high-profile mass shootings in recent years, gun control has proven to be an intractable debate within the US Congress, as lawmakers have failed to advance any significant policy changes to combat them.

Republicans and some moderate Democrats have resisted placing additional restrictions on gun ownership, and efforts to improve mental health services or establish new ways to identify potential shooters before they act have not gained traction.

Democratic leaders responded to the pair of shootings with a call for action.

They urged Republican Majority Leader Mitch McConnell to hold an emergency session to debate gun control legislation, after representatives left Washington just a few days ago for a five-week break.

Mr McConnell's office did not immediately respond to a request for comment.

Mourners gather at a vigil following the nearby mass shooting in Dayton, Ohio. ( AP: John Minchillo )

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