The heartbroken father of one of the young children killed in Tuesday’s school shooting in Texas has told how he learned his daughter had died while treating another girl for her injuries.

“One little girl was just covered in blood head-to-toe,” medical aide Angel Garza said.

“I asked her what was wrong, and she said she was OK. She was hysterical, saying they shot her best friend. ‘She’s not breathing, and she tried to call the cops’ [the girl said]. I asked her what’s her name and she said, she told me Amerie, she said Amerie.”

Ten-year-old Amerie Jo Garza was one of 19 children and two teachers to die when a teenage gunman burst into their classroom at Robb Elementary School in Uvalde, about 130 kilometres west of San Antonio, on Tuesday.

Mr Garza said his daughter was trying to help her classmates as 18-year-old Salvador Ramos opened fire.

“She was just trying to do the right thing,” he said. “She was just trying to call the cops, that’s all.”

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Amerie got a mobile phone for her birthday, just two weeks ago.

“She’d been wanting a phone for so long, and we finally got it for her. She just tried to call the police,” he told CNN.

“I just want people to know that she just died trying to save her classmates. She just wanted to save everyone.”

Mr Garza learned of his daughter’s bravery as he drove to the school to help the shooting victims. Two students later told him Amerie was trying to call police when she was shot.

“She was the sweetest little girl who did nothing wrong. She listened to her mom and dad. She always brushed her teeth. She was creative. She made things for us. She never got in trouble in school,” he said.

Ramos had posted on social media about his intentions, before he shot his grandmother and drove to the primary school. Once there, he barricaded himself in the classroom, firing an assault rifle at children and teachers.

Surviving students climbed through smashed windows to escape, while teachers Irma Garcia and Eva Mireles tried in vain to save the children.

Ms Garcia’s nephew John Martinez told The Washington Post that officials had said she helped to shield students from the gunfire.

“I want her to be remembered as someone who sacrificed her life and put her life on the line for her kids,” he said.

“They weren’t just her students. Those were her kids, and she put her life on the line, she lost her life to protect them. That’s the type of person she was.”

Ramos’ grandmother remains in a critical condition in a San Antonio hospital.

The Uvalde attack – America’s deadliest school shooting since a gunman killed 20 children and six adults at Sandy Hook Elementary in Newtown, Connecticut, in December 2012 – has sparked anger and grief across the US.

In a sign of the charged political atmosphere, Beto O’Rourke, the Democratic candidate challenging Texas Governor Greg Abbott in a November state election, interrupted a news conference to confront Mr Abbott about the state’s permissive gun laws, shouting, “You are doing nothing”.

He said it was “insane” that an 18-year-old was legally permitted to acquire a semi-automatic rifle and vowed to pursue gun restrictions.

Ramos reportedly bought the guns days after he turned 18.

“We can get that done if we had a governor that cared more about the people of Texas than he does this own political career or his fealty to the NRA,” he said, referring to gun-rights organisation the National Rifle Association.

But Mr Abbott said stringent gun laws did not prevent violence and that politicians should instead focus on mental health treatment and prevention.

In a prime-time address to the US late on Tuesday, President Joe Biden called for new gun restrictions.

“As a nation, we have to ask when in God’s name we’re going to stand up to the gun lobby,” he said, his voice rising.

But new legislation appears unlikely to pass in Washington. Virtually all Republicans in Congress oppose gun restrictions, and there was no sign the massacre would alter that position.

The NRA’s annual meeting starts on Friday in Houston – Texas’ most-populous city and some 475 kilometres east of Uvalde. Republicans including Mr Abbott, Texas US Senator Ted Cruz and former president Donald Trump are all scheduled to speak.

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Senator Cruz has already blamed an unlocked door for allowing the shooter access to the school.

“Have one door into and out of the school and have … armed police officers at that door,” he told Fox News on Wednesday.

“If that had happened, if those federal grants had gone to this school, when that psychopath arrived, the armed police officers could have taken him out. And we’d have 19 children and two teachers still alive.”

In a statement, the NRA expressed sympathy for the victims but said the event would go on as planned.

Mr Biden and first lady Jill Biden will travel to Uvalde soon to console the families of shooting victims.

“Jill and I will be travelling to Texas in the coming days to meet with the families to let them know we have a sense, just a sense, of their pain,” Biden said.

“Hopefully bring some little comfort to a community in shock and grief and trauma.”

Just over a week ago, Mr Biden visited Buffalo, New York, to console the families of 10 people killed at a supermarket there – nearly all of them black – by an avowed white supremacist.

“I’m sick and tired of it,” he said on Wednesday.

How far government can go in controlling access to firearms remains one of the most divisive issues in the United States.

It pits those who say restricting the availability of guns will save lives against those who maintain guns themselves are not the root cause of mass shootings and that the right to bear arms is protected by the US Constitution.

Biden urged the Senate to quickly confirm Steven Dettelbach – his nominee to head the Bureau of Alcohol, Tobacco, Firearms and Explosives – whose mission includes enforcing US gun laws.

Mr Dettelbach appeared at his Senate confirmation hearing on Wednesday.

Mr Biden also renewed his criticism of the gun lobby, which routinely fights attempts to impose stronger regulations on the industry.

“Where’s the backbone? Where’s the courage to stand up to a very powerful lobby?” he asked.

Shootings have become so commonplace in American schools that data shows a gun being fired almost every day this year on school property, according to the K-12 School Shooting Database at the Naval Postgraduate School’s Centre for Homeland Defence and Security.

-with AAP