Asian financial hub Singapore on Friday said it was scrutinizing trade in virtual currencies such Bitcoin, precious stones and metals to forestall new forms of illicit financing by criminals and terrorists.

In an inaugural report on money laundering and terrorist financing risks, the city-state said these sectors were identified for further study "as technology evolves and criminals become more sophisticated".

Activists Wednesday urged Singapore to ensure due process and address migrant worker grievances after the government said it would deport 53 South Asian laborers for their alleged involvement in a rare riot.

Home Affairs Minister Teo Chee Hean on Tuesday said the deportation of the men -- 52 Indians and one Bangladeshi -- as well as criminal charges filed against 28 other Indians would "send a strong signal that we will not tolerate actions by anyone which threaten law and order in Singapore".

Singapore on Thursday banned alcohol sales and consumption this weekend in a district where South Asian workers staged the city-state's first riot in more than 40 years.

Banning vendors from selling alcohol in a district known as Little India from Saturday morning to early Monday, the Singapore Police Force declared the zone a "proclaimed area" under the Public Order Preservation Act.

Singapore police have questioned nearly 4,000 foreign workers in a widening crackdown following the city-state's first riot in more than 40 years, officials confirmed Wednesday.

Three more Indian nationals were charged in court Wednesday with rioting, in addition to 24 compatriots charged a day earlier with the same offence, which is punishable by up to seven years in jail and caning.

Singapore on Tuesday filed charges against 24 Indian nationals who allegedly took part in its first riot for over 40 years, as officials and activists warned against inciting racial hatred over the incident.

The men face up to seven years in jail plus caning for the hour-long fracas on Sunday night, triggered when an Indian construction worker was struck and killed by a private bus in a district known as Little India.

Singapore faced shocking scenes of burning cars and littered streets Monday following a riot by South Asian workers in the worst outbreak of violence in more than 40 years in the tightly controlled city-state.

The hour-long fracas on Sunday night, triggered when an Indian construction worker was struck and killed by a private bus in the Little India district, left the nation questioning the causes behind the violence.

Hundreds of South Asian workers staged a riot in Singapore late Sunday following a road accident, leaving 10 police officers injured and at least six vehicles damaged, police said.

A police statement said the disturbance started in the congested Little India district when one person was killed after being hit by a bus. The victim's identity was not immediately disclosed.

Singapore, which has Southeast Asia's most modern military, said Monday it was acquiring two new submarines from German defense contractor ThyssenKrupp Marine Systems.

In a statement, the defense ministry said it signed a contract to acquire two "Type 218SG" submarines, which are projected to be delivered in 2020.

Singapore on Friday said it would not be drawn into confirming or denying allegations that it was part of a U.S.-led electronic spying network in Asia.

Foreign Minister K. Shanmugam said at a forum that neighbors Indonesia and Malaysia, which summoned Singapore's envoys this week over reports that such a network exists, are aware that the city-state has no intention to harm relations.

Malaysia summoned the high commissioner from neighboring Singapore on Tuesday over a media report that said the city-state helped facilitate U.S.-Australian surveillance in the region.

Foreign Minister Anifah Aman had already summoned the heads of the U.S. and Australian missions earlier in November in protest over reports that a vast U.S.-led surveillance network included a listening post in America's Malaysian embassy.