Official details remain sketchy about the circumstances of the late-morning shooting at Robb Elementary School in the town of Uvalde, Texas, about 137 kilometres west of San Antonio.

At least 18 students and three adults were killed by a lone gunman in the shooting, according to a state senator who said he was briefed by law enforcement.

Authorities said the suspect acted alone. A motive is not yet known.

“As a nation we have to ask: when in God’s name are we going to stand up to the gun lobby?” a visibly shaken Biden said at the White House shortly after returning from a five-day trip to Asia that was bookended by tragedy.

“When in God’s name will we do what we know in our gut needs to be done?”

With first lady Jill Biden standing by his side, Biden said: “I am sick and tired of it. We have to act.”

“Don’t tell me we can’t have an impact on this carnage,” he said.

Just two days before Biden left on his trip, he met with victims’ families after a hate-motivated shooter killed 10 Black people at a grocery store in Buffalo, New York.

The back-to-back tragedies served as sobering reminders of the frequency and brutality of a US epidemic of mass gun violence.

“These kinds of mass shootings rarely happen anywhere else in the world,” Biden said. “Why?”

He directed flags to be flown at half-mast until sunset on Saturday in honour of the victims in Texas.

It marks the deadliest shooting at a US primary school since the Sandy Hook Elementary School shooting in 2012 which killed 20 children and six adults.

Biden highlighted that in the 10 years since the Sandy Hook shooting, there has been “over 900 instances of gunfire reported in school grounds”.

Vice President Kamala Harris said earlier that people normally declare in moments like this, “our hearts break. But our hearts keep getting broken … and our broken hearts are nothing compared to the broken hearts of those families”.

“We have to have the courage to take action … to ensure something like this never happens again,” she said.

Shortly before landing in Washington, Biden spoke with Texas Governor Greg Abbott from Air Force One “to offer any and all assistance he needs in the wake of the horrific shooting”, White House communications director Kate Bedingfield tweeted.

Abbott said the suspect, identified as 18-year-old Salvador Ramos, was apparently killed by police officers responding to the scene.

Two officers were struck by gunfire, though the governor said their injuries were not serious.

The motive for Tuesday’s massacre, the latest in a string of seemingly random mass shootings that have become commonplace in the United States, was not immediately known.

Abbott said the suspect was believed to have abandoned his vehicle and entered the school armed with a handgun, and possibly a rifle, before opening fire.

Investigators believe Ramos shot and killed his grandmother before going to the school, CBS News reported, citing unidentified law enforcement sources.

“It is being reported that the subject shot his grandmother right before he went into the school,” Abbott told reporters.

“I have no further information about the connection between those two shootings.”

University Hospital in San Antonio said on Twitter it had received two patients from the shooting in Uvalde, a 66-year-old woman and a 10-year-old girl. Both were in a critical condition.

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