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The FBI has said it is aware of bomb threats sent to schools in Ohio and Texas and there is "no evidence" they are credible.

Investigating fake threats "drains law enforcement resources and diverts them from responding to an actual crisis", the FBI said.

The threats targeted public schools in the two states in recent days.

Some schools this week are ramping up security "out of an abundance of caution", one district said.

In Ohio, 52 school districts received emails, claiming to be from a Russian organisation, over the weekend.

Members from at least one school district contacted local authorities, who escalated the security issue to the FBI and the Joint Terrorism Task Force.

Schools districts were informed by federal officials that there was no credible threat.

"Based on the nearly identical language of the emails and the emails being sent to hundreds of schools nationwide, the Department of Homeland Security determined that these emails are hoax emails, otherwise known as 'swatting' emails... meant to incite panic," Josh Straus, technology coordinator for Ohio's Logan-Hocking district, said in a statement.

A superintendent from the Oak Hills Local School District said police conducted walk-throughs of their buildings and nothing was found, but added that the district will increase security in its schools on Monday.

The Alexander Local School District in Ohio wrote on Facebook that their schools would be on a two-hour delay on Monday "as a precaution", though the local sheriff's department "does not feel that the threat is credible".

The FBI said it will continue to work with local and federal agencies to "act upon information as it comes to our attention".

It urged the public to report any information to authorities.

On Friday, multiple public schools in Texas received similar emailed hoax threats, which were also soon deemed not credible, though some schools remain on heightened alert.

The Texas Education Agency told the ´óÏó´«Ã½ in an emailed statement that it has been in contact with law enforcement and will continue to collaborate and provide authorities with additional information as it becomes available.

In November, similar hoax emails containing bomb threats were sent to California schools.