SINGAPORE - Unauthorised drones caused flight delays and disruptions at Changi Airport on Monday night, just days after a previous incident saw 37 flights delayed.

The Civil Aviation Authority of Singapore (CAAS) said on Tuesday (June 25) in response to media queries that approximately 15 departures and three arrivals were delayed, and another seven flights were diverted, due to bad weather and unauthorised drone activities.

"As a precautionary measure, arrival and departure flights were carefully regulated for short periods of time between 8.07pm and 9.07pm," the authority said in a statement shortly after midnight.

The Straits Times had on Monday night reported that illegal drones caused several flights to be delayed or diverted, with Singapore Airlines, Garuda and AirAsia among the carriers affected. More than one drone was also sighted, with the drones reportedly not of recreational make, ST understands.

CAAS said it is investigating the matter.

"Members of the public are reminded that the authorities take a serious view of errant operations of unmanned aircraft which may pose threats to aviation or endanger the personal safety of others, and will not hesitate to take enforcement action against those who contravene regulations," it added.

"Offenders could face a fine not exceeding $20,000 or imprisonment for a term of up to 12 months, or both."

ST had reported that flight arrival and departure boards at Changi Airport's terminals began reflecting delays in arrival and departure times from around 9pm.

Social media users also reported delays, with one Twitter user Hana complaining of being stuck in Pekanbaru due to a drone sighting at Changi. Another user, GrudgeGriefer, tweeted that the flight her mother and brother were to arrive on was delayed.

Flight tracking site Flight Aware showed BatikAir flight ID7157 heading from Jakarta towards Singapore and then going into a holding pattern south of Batam, before it diverted west towards Pekanbaru, where it landed at 9.07pm local time (10.07pm Singapore time).

It took off again over an hour later, and landed at Changi at 12.19am on Tuesday. The site also showed Garuda Indonesia flight GA846 - which also operates as Singapore Airlines codeshare flight SQ5566 - bound for Singapore being diverted to land at Batam's Hang Nadim Airport at 8.34pm local time (9.34pm Singapore time).

That flight landed at Changi Airport at 11.18pm, more than two hours after its scheduled arrival time of 9.05pm.

AirAsia flight AK1729 from Penang that was scheduled to land at Changi at 9pm was diverted to Johor's Senai Airport where it landed at 9.25pm. AirAsia flight AK1727 from Penang that was scheduled to land at Changi at 10pm was also diverted to Senai.

Both flights eventually landed at Changi at around midnight.

Monday night's incident was the second time there were disruptions involving drones at Changi Airport in less than a week.

Unauthorised drones sighted on June 18 had delayed 37 flights and affected operations of one of Changi Airport's two runways for up to 10 hours, until the morning of June 19.

Under the Unmanned Aircraft (Public Safety and Security) Act, the flying of drones within 5km of airports or military airbases, or at altitudes above 200ft (61m), without a permit is an offence.

Drones have disrupted civil aviation near other major airports.

More than 1,000 flights were forced to be diverted or cancelled last December after drones were spotted around London's Gatwick Airport, with 140,000 people affected.

Drones also caused severe delays at London's Heathrow Airport in January.