LONDON — About 10 European flights were targeted by bomb threats yesterday, forcing the brief closure of a Warsaw airport and the scrambling of air force planes in Rome, authorities said.

No explosives were found, despite extensive searches on the ground after the planes all took off and landed safely in different European cities just several hours apart.

Irish budget carrier Ryanair appeared to be the worst hit, with four of its flights to different destinations being targeted.

London’s Metropolitan police confirmed it “was made aware of threats... against four aircraft due to depart from or arrive at Heathrow Airport” Tuesday, one of Europe’s busiest air hubs.

It gave no further details, but also said there was a further warning about a flight at Manchester Airport.

Following checks “no security issues were identified”, the Met police added.

Two bomb warnings targeted planes heading to the Polish capital Warsaw, including an SAS Scandinavian Airlines flight from Copenhagen and one from Heathrow operated by Polish airline LOT, Polish airport authorities said.

Aena, the Spanish airport operator, confirmed an emergency protocol was activated “due to a bomb alert” on a Ryanair flight which landed at 10:30 am (0930 GMT) from Manchester at Murcia in southeastern Spain. The plane arrived without problems.

Other Ryanair flights from Lisbon to Rome, Dublin to Rome, and Dublin to the Bulgarian capital of Sofia also received warnings.