US authorities warned Tuesday of possible copycat mass shootings after an 18-year-old gunman slaughtered 19 children and two teachers at a primary school in Texas, two weeks ago.

In an updated advisory, the Department of Homeland Security also highlighted the risk of violence tied to upcoming events including a Supreme Court ruling on abortion rights, the loosening of border controls, and the November Congressional elections.

"The United States remains in a heightened threat environment," DHS said.

"In the coming months, we expect the threat environment to become more dynamic as several high-profile events could be exploited to justify acts of violence against a range of possible targets."

It said targets of attack could include public gatherings, religious institutions, minority groups and ideological opponents.

It said recent attacks by men acting alone demonstrated the challenge of protecting vulnerable targets and also outlined the possibilities to potential attackers.

"Individuals in online forums that routinely promulgate domestic violent extremist and conspiracy theory-related content have praised the May 2022 mass shooting at an elementary school in Uvalde, Texas and encouraged copycat attacks," the DHS advisory said.

"Others have seized on the event to attempt to spread disinformation and incite grievances, including claims it was a government-staged event meant to advance gun control measures."

DHS also mentioned the attack in Buffalo, New York, in which a racist 18-year-old with an assault rifle murdered 10 African-Americans, and a 2019 attack in El Paso, Texas that targeted Hispanics.

"Both the Buffalo and El Paso attackers indicated they were inspired by the 2019 attacker of two mosques in Christchurch, New Zealand," DHS said.

The warning also said foreign groups like Al-Qaeda and the Islamic State Group, and people backed by the Chinese, Iranian and Russian government, could encourage and amplify violence and conspiracy theories to deepen divisions in US society.

"As the US 2022 mid-term elections approach, malign foreign actors could bolster their messaging to sow discord and influence US audiences in keeping with practices during previous election cycles," DHS said.