Media in southeast Texas report multiple people are dead and the gunman is "down" after a shooting in a church outside San Antonio.

A neighbour, shot at by a gunman who killed at least 26 people in a US church, reportedly returned fire, striking the shooter.

Children were believed to be among the dead and injured after the man, identified by CBS News as 26-year-old Devin Patrick Kelley, walked into the Baptist Charleston church in Sutherland Springs, San Antonio, Texas and started firing.

Twenty three were killed inside the church, two killed outside the church, one died later at hospital, authorities said. One of the victims was believed to be the 14-year-old daughter of the church's pastor.

SOCIAL MEDIA Children were believed to be among the dead and injured after the man, identified by CBS News as 26-year-old Devin Patrick Kelley, walked into the Baptist Charleston church in Sutherland Springs, San Antonio, Texas and started firing.

It was believed as many as 30 people were injured in the shooting.

Reporter Erica Hernandez talked to neighbours when she arrived at the church after the gunfire.

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REUTERS Emergency services mass outside the church where 26 people were shot dead.

Seen by neighbours, the gunman had first started shooting outside the church, before going inside, she said.

"They said he may have dropped anywhere from three to eight [ammunition] clips as he was firing," she told local television station KSAT.

"He had a lot of ammo on him, he had a mask and he was in full gear".

KSAT Texas Governor Greg Abbott confirms 26 people killed in US church shooting.

Neighbours outside the church ran for cover, with the gunman firing out toward people's homes, Hernandez told

"At that point another neighbour that lived a little further down returned fire toward him, and that's when they believe they hit him, and then he got back into his vehicle and left the scene," she said.

Some of the neighbours followed the gunman until he reportedly crashed his vehicle, Hernandez told KSAT.

The shooter was found dead in his car, police said.