Wilson County Sheriff Joe Tackitt told CNN that at least 20 people were killed.
The wife of the pastor of the the church said the couple's 14-year-old daughter was among those killed in the mass shooting.
Initial reports suggested at least one child, a 2-year-old, was among those shot.
Sherri Pomeroy, the wife of church pastor Frank Pomeroy, told NBC News that her husband was out of town at the time of the shooting.
The group Gays Against Guns NYC posted on Facebook saying, "Our nation's gun violence epidemic grows more and more virulent by the day", before reminding readers that the country's largest mass shooting in Las Vegas, which Trump said should not be politicized, occurred just over a month ago.
The official, who is not authorized to comment publicly, said authorities were still confirming the identity of the shooter at First Baptist Church of Sutherland Springs.
Megan Posey, a spokeswoman for Connally Memorial Medical Center, which is in Floresville and about 15km from the church, said "multiple" victims were being treated for gunshot wounds.
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Police, ambulances and helicopters swarmed to the small church 35 miles east of San Antonio.
According to the church's website, there was a worship service Sunday morning that began at 11 a.m.
Texas Gov. Greg Abbot said his thoughts and prayers were with the victims and that "the details of this horrific act are still under investigation". The FBI & law enforcement are on the scene.
US President Donald Trump tweeted from Japan, where he is on an Asian trip, that he was monitoring the situation.
Sen. Ted Cruz, R-Texas, tweeted: "Keeping all harmed in Sutherland Springs in our prayers and grateful for our fearless first responders on the scene". The official toll has not yet been verified by law enforcement or local officials.
The gunman's motive is unknown, and authorities have not said whether Sunday's incident was terror related.
The massacre comes just weeks after a sniper killed 58 people at an outdoor concert in Las Vegas, the deadliest mass shooting in modern USA history.