Police boosted security on trains in northeast France on Wednesday after receiving a tip about a suspected terrorist threat to the rail line between Basel, Switzerland and Luxembourg, police and prosecutors said.

The DST counterintelligence service opened an investigation into the suspected threat after Luxembourg tipped off France's Interior Ministry overnight, the Paris prosecutor's office said.

A member of France's Synergie Officier police union said police and train agents had been handed a flyer with information on physical characteristics of five Iranians suspected in connection with the threat.

A Luxembourg police spokesman confirmed the existence of a threat but declined to give further details. The press office at Swiss Railways, as well as cantonal police in Basel, said they knew nothing about a suspected plot.

The Paris prosecutor's office said investigators "were in an initial phase of verifying the information that was passed along." Police have made no arrests, the prosecutor's office said.