Unauthorized drone flying caused the second spate of delays and flight diversions in less than a week at Singapore’s Changi airport on Monday night, the city-state’s aviation authority said. Around 18 departures and arrivals were delayed and seven flights were diverted from the global transit hub due to “bad weather and unauthorized drone activities,” the Civil Aviation Authority of Singapore said in a statement on Tuesday. 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 unauthorized drone flying, disrupting 38 flights. It is against the law in Singapore to fly a drone within five kilometers (three miles) of an airport without a permit. Authorities are investigating. A surge in the availability of drones has become an increasing security concern for airports around the world. In December, drone sightings caused three days of travel chaos at London’s Gatwick airport, resulting in the cancelation or diversion of about 1,000 flights at an estimated cost of more than 50 million pounds ($64 million).