It's against the law in Singapore to fly a drone within 5 km of an airport (Representational)

Drone sightings have disrupted flights at Singapore's main airport for the second time in a week, authorities said, as the devices increasingly cause havoc for air traffic worldwide.

About 18 arrivals and departures were delayed and seven flights diverted at Changi Airport late Monday "due to bad weather and unauthorised drone activities", the Civil Aviation Authority of Singapore said.

The disruption lasted about an hour, it said.

Last week Changi, one of Asia's busiest hubs, closed one of its runways for short periods due to unauthorised drone flying, disrupting dozens of flights.

It is against the law in Singapore to fly a drone within five kilometres (three miles) of an airport without a permit.

Drones are increasingly disrupting flights at airports around the world. London's Gatwick was paralysed for 36 hours in December following repeated drone sightings, with tens of thousands of passengers affected.