More details are set to be released shortly.
"The people of Sutherland Springs need our prayers right now", Ryan wrote on Twitter.
Texas Attorney General Ken Paxton said in a statement that his office "stands ready to assist local law enforcement as needed".
US President Donald Trump, on a visit to Japan, tweeted his concern, saying he was "monitoring the situation". The FBI was also en route to the scene around 2 p.m.
Trump is in Japan as part of a 12-day, five-country Asian trip.
"From what I've heard, someone just walked in and started shooting", she said. Authorities say the attacker is dead.
Wilson County Commissioner Albert Gamez Jr told CNN that dozens of people had been killed. The hospital is in Floresville, Texas.
A neighbour said the shooting happened just before noon local time at a time when there were about 50 people at the service - if that number is correct then nearly the entire congregation have been killed or injured.
The First Baptist Church is a fixture in Sutherland Springs, an area home to fewer than 900 residents, according the 2010 Census.
It was not immediately clear if the church was holding services at the time of the shooting.
Sutherland Springs is a town with a population of roughly 362, and is located 35 miles east of San Antonio.
And it came just over two years after a white supremacist, Dylann Roof, entered a historically black church in Charleston, South Carolina, and shot nine people to death.