MINNEAPOLIS, Minnesota - March 11, 2010 - All schools across the busy midwestern U.S. city of Minneapolis were put on lockdown early Wednesday after a vague threat was posted on two social networking sites, a spokeswoman said.

The lockdown, which affected around 50 schools in the Minneapolis public school district, meant children were told to remain in their classrooms while access to outside visitors was strictly limited.

"We don't really know what this threat is, we are just working with the police right now," a spokeswoman for the district told AFP, adding it was an unprecedented move affecting all schools from elementary to high schools.

"There is no need for parents to pick up their children, this is just a precautionary measure," added the spokeswoman, who asked not to be named, saying officials would update the situation as they found out more.

The social networking sites were not immediately identified.

The Minneapolis Star Tribune daily said the district had some 34,000 students, and specified that police had notified the district of the threat.

Meanwhile, in Minneapolis’ twin city of St. Paul, the Cretin-Derham Hall Catholic School was also locked down, Mary Jo Groeller, the school's admissions administrator, told the Star Tribune.

An email threat noticed at the beginning of the school day "was very vague," she said, adding classes were going on as normal as the school was searched.

"You can't take this kind of thing lightly, but life's going on as usual," Groeller told the daily.