Helicopters and emergency personnel were arriving at the scene, and agents of the Bureau of Alcohol, Tobacco, Firearms and Explosives were heading to the scene, the bureau said. Other reports put the number of dead at 27.
The number of people killed and injured has not been confirmed by authorities, but children are believed to be among the victims.
A senior law enforcement source familiar with the investigation told NBC News that a single gunman opened fire at the First Baptist Church, injuring at least 15 people and killing several others.More news: George HW Bush calls Trump 'a blowhard' in new book
He was not at the church at the time of the massacre.
A law enforcement official says more than 20 people have been killed in a shooting at a church in a small town outside San Antonio.
Kelley reportedly entered the First Baptist Church in Sutherland Springs and began shooting Sunday afternoon. It is not known whether police shot the gunman or if he killed himself.
Sutherland Springs has a population of about 600 people and is located 34 miles east of San Antonio.More news: Sadio Mane helps Liverpool down West Ham
"Another church shooting. Lord have mercy", tweeted Russell Moore of the Ethics and Religious Liberty Commission of the Southern Baptist Convention.
Texas Gov. Greg Abbott called the shooting an "evil act", and promised "more details" from the state's Department of Public Safety soon.
Abbot is en route to Sutherland Springs to speak with law enforcement and the victims.
Carrie Matula, who works at a nearby gas station near, told MSNBC she heard what sounded like semiautomatic gunfire coming from the modest, white clapboard church with a red door. "I would never have thought this would have taken place here". "This is something that happens in a big city".More news: Unemployment Rate Falls to Lowest Point Since December 2000