By Jeremy Diamond, CNN

(CNN) -- President Donald Trump said Tuesday "there would have been no difference three days ago" in the deadly Texas shooting if an extreme vetting policy for gun ownership had been in place.

"If you did what you're suggesting there would have been no difference three days ago, and you might not have had that very brave person who happened to have a gun in his truck and shoot him, and hit him and neutralize him," Trump said responding to a question during a joint news conference with South Korean President Moon Jae-in.

"Instead of having 26 dead he would've had hundreds more dead," Trump said.

Trump has called for "extreme vetting" of immigrants from Muslim-majority countries in response to terrorist incidents. But he has resisted any action on gun laws in response to the deadly mass shootings that have occurred during his time in office.

Trump added that "the city with the strongest gun laws in our nation is Chicago. And Chicago is a disaster. A total disaster."

Sunday's church massacre where 26 people were killed took place in Sutherland Springs, Texas. It marks the deadliest shooting in Texas history, with victims ranging in age from 17 months to 77 years old.

Trump said Monday that he believes the Texas church shooting was caused by a "mental health problem," not an issue with gun laws in the United States.

"Mental health is your problem here," Trump said, noting that "based on preliminary reports" the shooter was "a very deranged individual."

"This isn't a guns situation," Trump said. "This is a mental health problem at the highest level. It's a very, very sad event."

"A very, very sad event, but that's the way I view it," Trump said.