A man armed with an assault rifle has opened fire at a packed shopping area in El Paso, in the US state of Texas, killing at least 20 people and wounding 26 others, officials said.

Authorities said on Saturday the suspect was arrested outside a Walmart store near the Cielo Vista Mall, eight kilometres from a border checkpoint with Mexico. 

Police confirmed that a 21-year-old from Allen, Texas, was the only person in custody. US media identified the suspect as Patrick Crusius, a white male.

Authorities said they were working to confirm whether a racist, anti-immigrant screed posted online shortly beforehand was written by the man who was arrested. The "manifesto" includes passages railing against the "Hispanic invasion" of Texas and the author makes it clear that he expected to be killed during his attack.

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

Speaking to reporters, Texas Governor Greg Abbott said: "Lives were taken who should still be with us today. Twenty innocent people from El Paso have lost their lives and more than two dozen more are injured."

He added: "We are going to aggressively prosecute it both as capital murder but also as a hate crime, which is exactly what it appears to be."

Shopping centre 'at capacity'

Mexico's President Andres Manuel Lopez Obrador said three Mexicans had been killed in the shooting and Foreign Minister Marcelo Ebrard said on Twitter six others were wounded.

El Paso in west Texas, which has about 680,000 residents, sits across the border from Juarez, Mexico.

Various news reports said the ages of victims being treated at hospitals ranged from two to 82 years. About half of the wounded, including at least two children, were initially taken to the University Medical Center of El Paso, where a number of the victims remained in surgery hours later, hospital spokesman Ryan Mielke said.

Anywhere from 1,000 to 3,000 shoppers and about 100 employees were at the Walmart store when the gunman opened fire, according to Robert Gomez, a local police sergeant. The shopping centre, described as being "at capacity" at the time of the shooting was particularly busy with customers buying goods for the upcoming school year.

US President Donald Trump called the shooting "an act of cowardice", writing on Twitter: "I know that I stand with everyone in this Country to condemn today's hateful act. There are no reasons or excuses that will ever justify killing innocent people."

The incident ranked as the eighth-deadliest mass shooting in modern US history, taking place six days after a gunman killed three people at a summer food festival in northern California.

A witness, who gave her name as Vanessa, said she had just pulled into the car park of the Walmart and "all of a sudden you heard what sounded like fireworks, really loud fireworks".

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

The witness told Fox that she had seen one man open fire wearing what appeared to be combat fatigues. "He was wearing a black T-shirt, camo-coloured pants [and] something to cover his ears, like headphones, really thick ones. "He was carrying a dark rifle and he was just pointing at people and just shooting and, yeah, the last thing I saw, he shot somebody that was in a corner." After seeing the woman fall in the car park, "that's when I thought, okay ... these aren't fireworks ... He was just shooting randomly. It wasn't to any particular person. It was any [person] that would cross path."