The United States was on heightened alert for school violence Friday after rumors of potential shootings spread on TikTok, though authorities said no credible threat had been found.

Some schools -- already on edge after a recent shooting -- sent warnings to parents and stepped up security, with a number of districts also canceling classes for the day.

"The White House and federal law enforcement are closely monitoring threats of violence in schools circulating on social media," White House Press Secretary Jen Psaki said on Twitter.

TikTok said that it was working with law enforcement to investigate reported videos discussing an upcoming school shooting, but had "found nothing."

"Local authorities, the FBI, and DHS (Department of Homeland Security) have confirmed there's no credible threat," the company said, "so we're working to remove alarmist warnings that violate our misinformation policy."

"If we did find promotion of violence on our platform, we'd remove and report it to law enforcement," it added.

The FBI said it was examining supposed threats, and the DHS said in a tweet that it did "not have any information indicating any specific, credible threats to schools."

- Recent school shootings -
Coming less than three weeks after the latest mass shooting in a US high school -- which left four students dead in Oxford, Michigan -- school administrators were quick to respond to the rumors.

"There is information circulating that today may be a day of attacks on schools. Administration and DC Police are very aware of this and on close watch," a message sent to parents by a school in Washington said Friday morning.

The Pennsbury School District in Pennsylvania told parents that there would be an increased police presence even though they "do not believe the threat to be credible."  

Psaki said that "today is another reminder of how many kids and parents live in fear of school shootings or violence. It is unacceptable."

TikTok, one of the most popular social media services with school-aged children, has already come under fire for allowing the posting of short videos challenging people to risky, damaging and illegal acts.

Earlier this year the viral "Devious Licks" challenge video encouraged students to vandalize bathrooms at schools and make copycat TikTok videos of it.

That led to numerous cases of minor destruction of school property around the country.

At the time TikTok said it was taking action to remove videos and minimize their spread "to discourage such behavior."

According to Everytown for Gun Safety, an anti-gun violence nonprofit, at least 149 school shootings have occurred in the United States so far this year, with 32 deaths and 94 injuries.

Sandy Hook Promise, a group formed in response to the 2012 mass shooting at Sandy Hook Elementary School in Newtown, Connecticut, condemned the alleged threat.

"Gun violence is not a subject for jokes or pranks. All threats need to be taken seriously," they said in a statement.