A string of bomb threats continued to target flights across Europe on Friday.A Ryanair flight travelling from Rome to Lisbon was placed under a security alert after an anonymous phone call to the control tower at Rome’s Fiumicino Airport warned of a possible explosive device on board, according to reports.The aircraft landed at Lisbon’s Humberto Delgado Airport shortly before 8 am. Authorities are investigating the threat, but there has been no confirmation of explosives on board.Meanwhile, the French Prime Minister’s office declared that there had been four more bomb alerts concerning “planes flying over French territory” on Friday morning.”The aircrafts landed safely. Security services are conducting inspections”, said the PM’s office, while adding that “an investigation is underway to determine the source of these alerts, which are affecting the whole of Europe.”Concerning the Rome-Lisbon flight, CNN Portugal reported that Portugal’s Public Security Police (PSP) received a telephone call from a person described as a “law enforcement officer”, warning of a possible explosive device on board. The caller’s identity was reportedly verified by the Judicial Police (PJ).When contacted by Euronews, Lisbon Airport referred enquiries to the PSP, which has not yet confirmed the reported incident. Euronews also attempted to contact the PJ but has not received a response.Another bomb alert was reported on a Wizz Air flight travelling from Prague to Rome. The aircraft was diverted to Venice, where it landed.The latest incidents follow a wave of bomb scares that affected at least 25 flights across Europe on Thursday. EU officials have said it is too early to determine whether the incidents are linked.