Singapore police said they arrested 66 suspected illegal immigrants from Malaysia and Thailand in a pre-dawn raid Wednesday on a forested hideout in the north of the city-state.

A police statement said 22 men and 39 women from Thailand and five Malaysian men were found in 20 makeshift wooden huts in a forested area the size of about four football fields in Woodlands, near the border with Malaysia.

"Some of those arrested were without valid documents and overstayers... investigations are in progress," the statement said.

"Through ground intelligence, the localised operation was aimed at flushing out immigration offenders who were residing in the said area and believed to be also engaged in vice activities."

Under Singapore law, illegal immigrants face up to six months' jail and a maximum fine of 6,000 Singapore dollars. They can also be caned. - AFP