An off-duty soldier who was inside an El Paso shopping centre next to the site of a fatal mass shooting has recounted how a child warned shoppers about a gunman minutes before he scooped up unattended minors and took them outside.

Key points: Army specialist Glendon Oakley was inside an El Paso shopping centre when 20 people were killed in a mass shooting

Army specialist Glendon Oakley was inside an El Paso shopping centre when 20 people were killed in a mass shooting He says a child ran into the store he was shopping at warning of an active shooter, then five minutes later he heard gunshots

He says a child ran into the store he was shopping at warning of an active shooter, then five minutes later he heard gunshots The off-duty soldier then picked up unattended children who were running around the shopping centre to carry outside

Twenty people were killed and 26 others seriously injured in the shooting in the Texas border town.

Army specialist Glendon Oakley was at a sports store in the Cielo Vista Mall when a "little kid" ran in warning shoppers and retail workers there was an active shooter at the nearby Walmart, Mr Oakley told local media outlets.

"A little kid ran in there telling us 'there's an active shooter in Walmart', but we didn't pay it no attention because for one it's a just little kid and for two we're at the mall and not at Walmart," he told CNN.

The Walmart where the suspect started shooting is across the road from the Cielo Vista Mall, where witnesses heard gunshots and ran from the centre. ( Google Maps )

Mr Oakley said about five minutes later he had walked to another store in the centre when he heard gunshots and saw people start running and screaming.

His first thought was to follow another group of shoppers to the car park so he could protect them.

"I'm in the military so when I hear gunshots we're trained to think fast. Grab your weapon, think fast, take cover, do anything you can," he said.

"But I see a whole bunch of kids just running around without their parents. So … the only thing I think of is just to pick up as many kids as possible."

Mr Oakley said he saw about 13 children without adults, but he was only able to pick up three. He said another man also took three children away to safety.

"There was maybe a total of 13 kids running around but I can only get three, and I think he got about three as well," he said.

"I was just focused on the kids I wasn't really worried about myself. I put my head down and just ran as fast as I could.

"And once I saw a whole bunch of cops I dropped [the kids] and I told them to go."

Shoppers fled to the car park after hearing gunshots inside the shopping centre. ( Reuters: Jorge Salgado )

Mr Oakley said when he saw police officers, he worried they might shoot at him thinking he may have been the active shooter.

"So I pulled out my phone in case they were going to shoot me … and started recording while I was running," he said.

"They were telling me 'hands up, hands up'."

He said he showed officers his licence to carry and he was allowed to leave.

"I wasn't too worried about myself I was just trying to get those kids out of there," he said.

"I was just thinking about if I had a child and if I wasn't around, how I would want another man to act if they saw my child just running around?"

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 49 seconds 49 s Many of those caught up in the Texas shooting were buying back-to-school supplies

Sergeant Robert Gomez said most of the El Paso attack victims were shot at a Walmart near the Cielo Vista Mall.

He said the store was packed with back-to-school shoppers, with anywhere from 1,000 to 3,000 customers and about 100 employees, when the attack happened.

A suspect is in custody over the shooting, with police alleging the 21-year-old man used a rifle in the attack. He was arrested away from the scene of the shooting without police firing a shot.

El Paso Police Chief Greg Allen said a manifesto possibly written by the suspect indicated the shooting may have "a nexus to a hate crime".

Two-year-old and nine-year-old among injured

El Pasoans lined up at local blood banks to donate to victims of the mass shooting. ( AP: Rudy Gutierrez )

Two of the patients who arrived at the University Medical Centre of El Paso were children, aged two and nine, hospital spokesman Ryan Mielke told CNN.

They had non-life-threatening injuries and were transferred to El Paso Children's Hospital.

Hospitals in the area were overwhelmed by the response to an appeal for blood donations, with lines around the block at some clinics and would-be donors being turned away.

"We need to pull together and help people in need," El Paso local Carlos Flores said.

Loading

ABC/wires