Four women - three Russians and an Uzbek - were arrested by the police on May 4 for their alleged involvement in a "high-class" prostitution ring.

The women, aged between 26 and 39, were said to have valid work permits and had supposedly advertised their $500-an-hour services on at least 10 online platforms.

Yet, overall figures depict a declining trend of foreign women caught for vice-related activities.

Last year, 2,947 foreign women were arrested for suspected vice activities, the police told The Sunday Times. In 2015 and 2014, the figures were 4,976 and 4,886 respectively.

Foreigners like Anna, 26, the Russian escort masquerading as a student, will not be allowed to remain here upon conviction or cancellation of their passes, the Immigration and Checkpoints Authority (ICA) told The Sunday Times.

A spokesman said: "The ICA takes a serious view of international students who abuse their student's pass status to work illegally or engage in criminal or undesirable activities in Singapore. Those found doing so will have their passes cancelled and may also be charged in court, depending on the nature of the offence."

In the recent case, a Russian woman, 39, has been charged with three counts of knowingly living in part on the earnings of two foreign prostitutes.

For each charge, she faces a jail term of up to five years and a fine of up to $10,000.

Zaihan Mohamed Yusof