A gunman with an assault rifle stormed a church in rural Texas on Sunday, killing at least 26 worshippers and wounding 20 others in a rampage to be notched into a litany of mass shootings that have plagued the United States in recent years.

The lone suspect, dressed in black tactical gear and a ballistic vest, drove up to the white-steepled First Baptist Church in Sutherland Springs during Sunday morning services and started firing inside. He kept shooting once he entered, killing or wounding victims ranging in age from 5 to 72 years old, law enforcement officials told a news conference.

US President Donald Trump later today called the mass shooting in Texas that left at least 26 people dead a "mental health problem," not "a gun situation."

Asked what policies he might support in response to the shooting at a press conference in Tokyo, Trump said that based on preliminary reports, the gunman was "a very deranged individual, a lot of problems".

He said Sunday's attack "isn't a guns situation" but "a mental health problem at the highest level. It's a very, very sad event".

Among the dead was the 14-year-old daughter of Pastor Frank Pomeroy, the family told several television stations.

The gunman was later found dead of a self-inflicted gunshot wound, Wilson County Sheriff Joe Tackitt told CBS News in an interview this morning.

Tackitt said gunfire was exchanged between the gunman and two armed citizens during a vehicle chase after the shootings.

"There was some gunfire exchanged, I believe, on the roadway also, and then (the shooter's vehicle) wrecked out," said Tackitt. "At this time we believe that he had a self-inflicted gunshot wound."

Texas Governor Greg Abbott said at the news conference: "We are dealing with the largest mass shooting in our state's history. The tragedy of course is worsened by the fact that it occurred in a church, a place of worship where these people were innocently gunned down."

The massacre appeared to shake the close-knit community to its core. About 65km east of San Antonio in Wilson County, Sutherland Springs has fewer than 400 residents.

"This would never be expected in a little county like Wilson County," Texas Attorney General Ken Paxton told CNN.

After leaving the church, the gunman was fired on by a local resident with a rifle. The suspect dropped his Ruger assault weapon and fled in his vehicle, said Freeman Martin, regional director of the Texas Department of Public Safety.

Soon afterward, the suspect crashed the vehicle near the border of a neighboring county and was found dead inside with a cache of weapons. It was not immediately clear if he killed himself or was hit when the resident fired at him outside the church, authorities said.

The suspect's identity was not disclosed by authorities, but law enforcement officials who asked not to be named said he was Devin Patrick Kelley, described as a white, 26-year-old man, the New York Times and other media reported.

"We don't think he had any connection to this church," Wilson County Sheriff Joe Tackitt told CNN. "We have no motive."

Jeff Forrest, a 36-year-old military veteran who lives a block away from the church, said what sounded like high-caliber, semi-automatic gunfire triggered memories of his four combat deployments with the Marine Corps.

"I was on the porch, I heard 10 rounds go off and then my ears just started ringing," Forrest said. "I hit the deck and I just lay there."

The massacre came just weeks after a sniper killed 58 people at an outdoor concert in Las Vegas, the deadliest mass shooting in modern US history.

The shootings have stirred a years-long national debate over whether easy access to firearms was contributing to the trend.

In rural areas like Sutherland Springs, gun ownership is a part of life and the state's Republican leaders for years have balked at pushes for gun control, arguing that more firearms among responsible owners makes the state safer.

To honor the victims, President Donald Trump ordered flags on all federal buildings to be flown at half staff.

"This act of evil occurred as the victims and their families were in their place of sacred worship," the president said in Tokyo during the first leg of a 12-day Asian trip.

"Through the tears and through the sadness we stand strong, oh so strong."

While authorities provided little information about the suspect, online records show that a man named Devon Patrick Kelley lived in New Braunfels, Texas, about 56km north of Sutherland Springs.

The US Air Force said Kelley served in its Logistics Readiness unit at Holloman Air Force Base in New Mexico from 2010 until his discharge.

Kelley was court-martialed in 2012 on charges of assaulting his wife and child, and given a bad-conduct discharge, confinement for 12 months and a reduction in rank, Air Force spokeswoman Ann Stefanek said.

Kelley's Facebook page has been deleted, but cached photos show a profile picture where he appeared with two small children. He also posted a photo of what appeared to be an assault rifle, writing a post that read: "She's a bad bitch."