Thirty people died and dozens were wounded in two mass shootings within just 13 hours of each other in the United States, shocking the country and prompting calls from some politicians for tighter gun control.

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

Texas Governor Greg Abbott said the rampage appeared to be a hate crime, and police cited a "manifesto" they attributed to the suspect, a 21-year-old white man, as evidence that the bloodshed was racially motivated.

Across the country, a gunman opened fire in a downtown district of Dayton, Ohio, early on Sunday, killing nine people and wounding at least 26 others, police and the city mayor said. The assailant was shot dead by police.

In the Texas border city of El Paso, a gunman opened fire on Saturday morning in a shopping area packed with thousands of people during the busy back-to-school season, killing 20 and injuring more than two dozen, many of them critically. The shooting was being investigated as a possible hate crime as authorities worked to confirm whether a racist, anti-immigrant screed posted online shortly beforehand was written by the man arrested in the attack on the 680,000-resident border city.

Just hours later in Dayton, Ohio, a gunman wearing body armour and carrying extra magazines opened fire in a popular nightlife area, killing nine and injuring at least 26 people.

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As President Donald Trump led the condemnation over "an act of cowardice" that police are treating as a possible hate crime, calls rang out to end the "epidemic" of gun violence.

It was the second fatal shooting in less than a week at a Walmart store in the US and comes after a mass shooting in California last weekend.

One suspect was taken into custody while authorities were studying an extremist manifesto purportedly written by the gunman.

Footage shot on camera phones appeared to show multiple bodies lying on the ground in the store's parking lot while other footage showed terrified shoppers running out of the store as gunfire echoed.

Two to 82 years

"Twenty innocent people from El Paso have lost their lives," Texas Governor Greg Abbott told a press conference.

"We as a state unite in support of these victims and their family members... We pray that God can be with those who have been harmed in any way and bind up their wounds."

Police is seen after a mass shooting at a Walmart in El Paso, Texas, US. Jose Luis Gonzalez/Reuters

Police chief Greg Allen confirmed that in addition to the 20 confirmed fatalities, there were 26 wounded.

Mexican President Andres Manuel Lopez Obrador said three Mexican citizens were killed and Foreign Minister Marcelo Ebrard said on Twitter six others were wounded.

Various news reports said the ages of victims being treated at hospitals ranged from two to 82 years.

Police said that Walmart was "at capacity" at the time of the shooting, with 1,000-3,000 customers inside.

Police confirmed that a 21-year-old from Allen, Texas, was the only person in custody. US media identified him as Patrick Crusius, who is white. He surrendered to police about a block away from the Walmart.

"Right now we have a manifesto from this individual that indicates to some degree, it has a nexus to potential hate crime," Allen said.

The "manifesto" purportedly written by Crusius that was circulated online includes passages railing against the "Hispanic invasion" of Texas and the author makes clear that he expected to be killed during his attack.

Texas Governor Greg Abbott speaks during a press briefing, following a mass fatal shooting in Texas. Joel Angel Juarez/AFP

Veronica Escobar, who represents El Paso in Congress, said "the manifesto narrative is fueled by hate."

"And it's fueled by racism and bigotry and division," she said at a press conference with El Paso Mayor Dee Margo and Governor Abbott.

Shoppers exit with their hands up after a mass shooting at a Walmart in El Paso, Texas, US. Jorge Salgado/Reuters

Witnesses said the gunman appeared to be shooting at random when he opened fire around 10:30 am.

One woman, who gave her name as Vanessa, said she had just pulled into the Walmart parking lot when the shooting began.

"You could hear the pops, one right after another and at that point as I was turning, I saw a lady, seemed she was coming out of Walmart, headed to her car. She had her groceries in her cart and I saw her just fall," she told Fox News.

The witness said the gunman wore black T-shirt, combat trousers and was wearing ear muffs.

"He was just shooting randomly. It wasn't to any particular person. It was any that would cross paths."

People gather in Juarez, Mexico, in a vigil for the 3 Mexican nationals who were killed in an El Paso shopping-complex shooting.

Another shopper described how he managed to avoid being hit by hiding along with his mother between two vending machines just outside the store.

"That's where the individual tried to shoot at me, which he missed cause I kind of ducked down," Robert Curado told the El Paso Times.

"He had an AK-47."

'How you doing, brother'

Video captured by a witness in the parking lot in the immediate aftermath of the shooting showed three people lying motionless on the ground.

One had fallen next to a truck, while two were on the sidewalk outside the store entrance. " Ambulance! Help!" people cried as they rushed to the victims.

"How you doing, brother, how you doing," one man was heard saying on the recording.

A still captured from CCTV showed the gunman carrying what appeared to be an AK-47.

People hug at St Pius X Church at a vigil for victims after a mass shooting in El Paso, Texas on Saturday. AFP

Trump, who is spending the weekend at his golf club in New Jersey, said the shooting was "not only tragic, it was an act of cowardice".

"There are no reasons or excuses that will ever justify killing innocent people," he said on Twitter.

It has been a particularly bad week for gun violence in the United States.

Two people died and a police officer was wounded Tuesday at a Walmart in Mississippi.

Last Sunday a 19-year-old gunman opened fire at a food festival in northern California, killing three, including two children.

Beto O'Rourke, a former US congressman for El Paso who is now running for president, cut off his campaigning in the wake of the shooting.

"I'm incredibly saddened and it's very hard to think about this. But I tell you El Paso is the strongest place in the world, this community is going to come together," he told supporters.

Elizabeth Warren, a senator who is among the frontrunners for the Democratic party's presidential nomination, said "far too many communities have suffered through tragedies like this already.

"We must act now to end our country's gun violence epidemic," she said.

Another presidential hopeful Cory Booker said the US had "to end this national nightmare" and "find the moral courage to take action to end this carnage."

Agence France-Presse