A local sheriff, Joe Tackitt, earlier stated that an undetermined number of people had been killed, and that many had been taken to the hospital.
Local media report a two-year-old is among the shooting victims.
Contacted by Al Jazeera, the Wilson County sheriff was not immediately available for comment.
A law enforcement official who was briefed on the investigation told The Associated Press that more than 20 people were killed in the shooting. Paddock subsequently killed himself.
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The FBI and Texas Rangers are reportedly on the scene.
The shooter is dead, police said, and there is no active threat on the scene.
"May God be w/ the people of Sutherland Springs, Texas", tweeted President Donald Trump, who is in Japan on his first trip to Asia as president. "I am monitoring the situation from Japan", Trump tweeted.
Carrie Matula, who works at a service station about 140m away from the church, told MSNBC she heard "semiautomatic gunfire" and looked to see what was going on.
Sen. Ted Cruz, R-Texas, tweeted: "Keeping all harmed in Sutherland Springs in our prayers and grateful for our courageous first responders on the scene".