At least seven flights and a cruise ship in Europe received bomb threats on Tuesday, causing disruption at airports although no devices were discovered.

At least two flights to Warsaw, two to Sofia, two to Rome and one to Murcia airport in Spain were subjected to the threats on Tuesday, and another flight to Sofia had been threatened a day earlier.

An airport in Warsaw was briefly shut, and the Italian Air Force accompanied one of the flights to Rome while an Israel-bound cruise ship was searched for bombs in Greece.

There was no claim of responsibility and officials did not attribute blame. None of the affected airlines responded to Reuters’ requests for comment.

Rhys Jones, Analysis Output Manager at aviation risk advisory firm Osprey Flight Solutions, said that seven confirmed bomb threats in a single day affecting flights was exceptional.

“The lack of clear target state, airline, or route indicates that general disruption to aviation operations is the aim,” Jones said.

Authorities diverted a LOT flight from London that was meant to land at Warsaw’s Chopin airport to nearby Modlin airport, which was closed and then reopened after the plane was searched and no device was found.

Another Warsaw-bound flight, an SAS flight from Copenhagen, landed at Chopin as scheduled. Chopin airport said no device was found and the airport remained operational.

Bomb threats were made against planes travelling from Milan/Bergamo and Dublin to Sofia, Bulgaria’s transport ministry said in a statement. Both landed safely and no explosive devices were found on inspection.

The Bergamo flight was operated by Wizz Air and was carrying 176 passengers, the statement said. The Dublin flight, operated by Ryanair, had 186 passengers on board.

Bulgarian news agency BTA had earlier quoted an Interior Ministry official as saying a flight from Israel had been affected, but authorities later said this was not the case.

A day earlier, a Wizz Air flight from London to Sofia was diverted to Budapest airport after receiving a bomb threat, Hungarian Defence Minister Romulusz Ruszin-Szendi said in a Facebook post.

In Murcia in southeastern Spain, parts of an airport were cordoned off on Tuesday after a bomb threat was reported on a Ryanair flight from Manchester.

Officials said the report proved to be a hoax, and the airport resumed normal operations after authorities found no explosives on board. Ryanair declined to immediately comment.

Two other Ryanair flights – one from Lisbon and another from Dublin – were searched at Rome’s Fiumicino airport following bomb threats, said a source from airport operator ADR. The Dublin flight was escorted by the Italian Air Force until it landed.

The alert was cleared and the airport continued to operate as usual with no effects on other flights, the person said.

An explosives detection dog and a diver searched the Panama-flagged Crown Iris cruise ship on the Greek island of Syros after an unknown person said an explosive device had been attached to the boat’s hull, authorities said.

Initial checks found no explosive device with coastguard specialists expected to carry out further inspections before the ship could resume its voyage.

While no country pointed fingers over the bomb threats, they come at a time when European officials have been blaming both Russia and Iran for what they describe as “hybrid attacks” and acts of sabotage. Moscow and Tehran both deny any such campaign.

This week, unidentified hackers shut down fridges in supermarkets in several European countries. Some experts said hoax bomb threats could be part of a similar pattern of low-cost, deniable disruption efforts.

“From railways to the food infrastructure that you see happening with the fridges being hacked and all kinds of other operations, it does fit the narrative of Russian state actors who want to disturb Europe,” said Eric Schouten, the head of aviation security group Dyami.