SINGAPORE - There was a buzz in the night sky as a flash of green zipped over a forested area in Bukit Panjang. The unmanned aerial vehicle circled the area for nearly an hour.

On the ground, two men watched images on screens as they manoeuvred the drone remotely.

They were not ordinary drone enthusiasts but men in blue on a mission to find any illegal immigrants who might be hiding out in the thick vegetation surrounding Bukit Timah Expressway (BKE) on Wednesday night (Oct 24).

Based on intelligence received, the police had raided a hideout under a section of the expressway earlier that night. They arrested five men suspected to be immigration offenders from Indonesia and seized a large amount of contraband cigarettes.

A drone was then sent in to scan the surrounding area to detect any possible runaways who might have been hiding in the forest.

The use of drones is part of an ongoing trial by the Singapore Police Force (SPF) to see how technology can complement police operations.

A drone that is equipped with thermal-imaging capabilities enables the police to quickly spot people in a forested area .

On a screen attached to the drone's remote control, a person shows up as a red figure due to body temperature, while a cool forest appears blue.

The drone operation on Wednesday - which was witnessed by the media - took about an hour. No runaway was detected.

The arrest of the five men was part of a five-day enforcement operation led by the police and supported by agencies like the Central Narcotics Bureau, Immigration and Checkpoints Authority, Singapore Civil Defence Force and Singapore Customs.

The operation, which ended on Friday night, covered multiple locations in the western and northern parts of Singapore, including Tuas, Jurong West, Choa Chu Kang, Bukit Batok and Woodlands.

A total of 125 people - 62 men and 63 women- aged between 18 and 62 were arrested, said the police in a statement on Saturday.

A total of 4,332 packets of duty-unpaid cigarettes were seized. The total duty and goods and services tax (GST) evaded amounted to $43,980 and $3,190 respectively.

Assistant Commissioner Devrajan Bala, Commander of Jurong Police Division, thanked officers from the division and various agencies for their support and collaboration.

He said: "This joint operation is part of police's continuous efforts to clamp down on criminal activities. While the police works closely with other Home Team and law enforcement agencies to deal with those who take part in illicit activities, we also urge the public to be vigilant and not fall prey to crime."

The suspected offences of those arrested include vice-related activities in residential units, employment-related offences at public entertainment outlets, e-commerce scams, loansharking activities and immigration-related offences.

Investigations are ongoing.