At least 26 people have been killed and 20 injured after a gunman opened fire at a church in Sutherland Springs in the US state of Texas during Sunday services, media reports said.

The gunman, clad in all black, with a ballistic vest strapped to his chest and a military-style rifle in his hands,  entered the church and opened fire at around 11:30 local time (17:30 GMT), BBC reported.

The gunman was identified as Devin Patrick Kelley, 26, The New York Times reported, citing two law enforcement officials who spoke on the condition of anonymity.

Kelley, who lived in New Braunfels, Texas, and had served in the Air Force at a base in New Mexico, died shortly after the attack.

Governor Greg Abbott confirmed the death toll and said it was the worst mass shooting in the history of Texas. "This will be a long, suffering mourning for those in pain," he said at a news conference on Sunday.

The ages of the victims ranged from 5 to 72 years old, BBC quoted Texas Department of Public Safety regional director Freeman Martin as saying.

A local citizen "grabbed his rifle and began shooting" at the suspect, before the gunman fled in a vehicle.

The citizen pursued the suspect, who drove off the road and crashed his car at the Guadalupe County line. Police found the suspect dead in his car, but it is unclear if he died from a self-inflicted gunshot wound or from returned fire from the citizen, Martin added.

President Donald Trump, on a tour of Asia, called the slaying “horrific” and "act of evil".

"And through the tears and through the sadness, we stand strong, oh so strong," he added, stressing that Americans would pull together.