Two mass shootings in the US have left 29 people dead within 24 hours, the latest such attacks in a nation torn over how to tackle gun violence.

A gunman armed with an assault rifle killed 20 people Saturday when he opened fire on shoppers at a packed Walmart store in El Paso, Texas.

Less than 13 hours later, a lone shooter killed nine people in Dayton, Ohio early Sunday before being shot dead by responding police officers, the authorities did not disclose the shooter's identity.

The attacker opened fire around 1:00 am (0500 GMT) on a street in the popular bar and nightlife district called Oregon, leaving nine dead and 26 wounded.

Dayton Mayor Nan Whaley said the shooter was wearing body armor and had high-capacity magazines and extra magazines.

"In less than one minute Dayton first responders neutralized the shooter," she said.

"I am amazed by the quick response of Dayton police that saved literally hundreds of lives," Whaley said, adding that it was "the 250th mass shooting in America."

Actually, it was the 251st, according to the Gun Violence Archive, an NGO. It defines mass shooting as an incident in which at least four people are wounded or killed in a shooting.

"Fortunately we had multiple officers in the immediate vicinity when this incident started so there was a very short timeline of violence, for that we've very fortunate," he said, praising the swift response by officers.

"As bad as this is, it could have been much, much worse."

Carper said the attack appeared to be the work of a lone shooter.

The incident came just hours after the mass shooting at a Walmart store in Texas which has reignited debate about the US epidemic of gun violence.

President Donald Trump described the El Paso attack as "an act of cowardice" and police are treating it as a possible hate crime.

Saturday's suspect was officially identified as a 21-year-old white male from Allen, Texas, a Dallas suburb some 1,046 kimometers east of El Paso, which lies along the Rio Grande, across the US-Mexico border from Ciudad Juarez.

Footage shot with cellphones 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.

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