LONDON: About 10 European flights were targeted by bomb threats on Tuesday, 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.