Donald Trump has said the worst mass shooting in the modern history of Texas is not "a guns situation".

The US President spoke after a gunman walked into a church in a small town near San Antonio and opened fire, killing at least 26 people between the ages of five and 72.

The suspect — named as 26-year-old Devin Patrick Kelley — fled in his car after he was shot at by a local resident of Sutherland Springs. He was later found dead from apparent gunshot wounds in a neighbouring county, with a cache of weapons in his vehicle.

The massacre has sparked renewed calls for tighter gun control laws in the US.

Asked at a press conference in Tokyo what policies he might support in response to the shooting, Mr Trump said preliminary reports suggested the gunman was "a very deranged individual, [with] a lot of problems."

"We have a lot of mental health problems in our country, as do other countries. But this isn't a guns situation," he said. "Fortunately somebody else had a gun that was shooting in the opposite direction."

"This is a mental health problem at the highest level," he added. "It's a very, very sad event."

Authorities have not yet determined Kelley's motive and have not broached the topic of the gunman's mental health.

Earlier this Mr Trump signed a bill blocking plans that would have prevented an estimated 75,000 people with mental health disorders buying guns. The proposals were part of former president Barack Obama's push to strengthen the federal background check system in the wake of the 2012 shootings at Sandy Hook Elementary School shooting in Newtown, Connecticut.

Democrat senators for Connecticut have since led calls for gun control in Congress and issued impassioned pleas for action following the latest massacre on Sunday.

Law enforcement officials gather near the First Baptist Church in Sutherland Springs (Getty Images)

Republicans in Congress have long resisted calls from Democrats for tighter restrictions on firearms ownership. After Stephen Paddock killed 58 people in Las Vegas in the deadliest mass shooting in US history last month, White House press secretary Sarah Huckabee Sanders said it was "premature" to discuss gun policy.

Barack Obama was among the Democrat politicians who called for change following the Sutherland Springs massacre.

The former President urged the US to "ask what concrete steps we can take to reduce the violence and weaponry in our midst".

Connecticut senator Chris Murphy said Republican lawmakers "need to think about whether the political support of the gun industry is worth the blood that flows endlessly onto the floors of American churches, elementary schools, movie theatres and city streets."

He added: "As long as our nation chooses to flood the country with dangerous weapons and consciously let those weapons fall into the hands of dangerous people, these killings will not abate."

"Enough is enough," said Richard Blumenthal, also a senator for Connecticut. "Now is the time for commonsense gun violence prevention steps."

He added: "Prayers are important but insufficient. After another unspeakable tragedy, Congress must act - or be complicit."

Pennsylvania senator Bob Casey said Congress "must take action on gun violence", while Dianne Feinstein, senator for California, said: "When will this end? When will we decide that we can’t accept massacres in our places of worship, schools, or at concerts? When will we actually do something about it?

“This latest mass shooting comes just one month after the country’s deadliest mass shooting, and we’re still trying to garner support for even the most basic steps to reduce gun violence."

Law enforcement officials investigate the scene at the First Baptist Church in Sutherland Springs, Texas (Reuters)

A sheriff has said there was likely "no way" for parishioners to escape the church once shooting began. Wilson County Sheriff Joe Tackitt Jr said Kelley fired several shots outside the First Baptist Church before walking inside towards the front of the congregation.

The gunman, dressed in black tactical-style gear and armed with an assault rifle, then turned around and opened fire on his way back out the front door.

"I don't think they could have escaped," said Sheriff Tackitt Jr. "You've got your pews on either side."

Kelley dropped his Ruger assault weapon and fled in his car after being fired at by a member of the public. Two residents pursued him before he lost control of his vehicle and crashed. He was found dead in his car, but it is not clear whether his fatal injuries were self-inflicted or caused when he was shot at.

Neighbours of an address listed for Kelley in a suburb of San Antonio have said they heard intense gunfire from the direction of the house in the last few days.

"It's really loud," said Ryan Alberts, 16, who lives across the road. "At first I thought someone was blasting."

He added: "It was someone using automatic weapon fire."

Mourners at a candlelight vigil for the victims of the shooting at the First Baptist Church (AP)

The US Air Force said Kelley was a former serviceman. He served in logistics readiness at Holloman Air Force Base in New Mexico from 2010 before being court-martialled in 2012 and dishonourably discharged two years later.

Air Force spokeswoman Ann Stefanek said Kelley was court-martialled for assaulting his wife and child. He received a bad conduct discharge, 12 months confinement and a reduction in rank.

US officials said Kelley did not appear to have links to any organised terror groups. Investigators are looking at Facebook posts he made in the days before the massacre, including one which appeared to show an AR-15 semi-automatic weapon.

"We are dealing with the largest mass shooting in our state's history," Texas Governor Greg Abbott said a news conference. "The tragedy, of course, is worsened by the fact that it occurred in a church, a place of worship."

Sutherland Springs, about 40 miles (65km) east of San Antonio, has fewer than 400 residents.

"This would never be expected in a little county like [this]," Texas Attorney General Ken Paxton told CNN.

Among the dead was the 14-year-old daughter of Pastor Frank Pomeroy.

Sunday's massacre occurred on the eighth anniversary of a shooting in which 13 people died at at Fort Hood Army base in central Texas. A US Army Medical Corps psychiatrist convicted of the murders is awaiting execution.

In 2015, a white gunman killed nine black parishioners at Emanuel African Methodist Episcopal Church in Charleston, South Carolina. The gunman was sentenced to death for the racially motivated attack.