Singapore's central bank and white-collar crime police are probing several broking firms and trading representatives as part of an investigation into possible breaches of the securities act.

The Securities and Futures act relates to the regulation of activities and institutions in the securities, futures and derivatives industry, and of clearing facilities.

"As investigations are ongoing, we are not able to provide further information," the Monetary Authority of Singapore said in a statement.

It said it obtained documents from the broking firms and trading representatives in the joint investigation with the Commercial Affairs Department of Singapore's police.

Singapore has taken steps to improve its scrutiny on trading activities following a penny stock scandal, which saw share prices of three stocks crash spectacularly in 2013 after huge run-ups turned them briefly into billion-dollar firms.

This took a toll on stock trading volumes at the Singapore Exchange Ltd, which is also the city-state's market regulator.

In a separate emailed statement in response to Reuters queries on Friday, SGX said it had referred three cases of insider trading and six cases of market manipulation to the MAS in January to March this year. The SGX said it regularly refers case investigations to the MAS.

Last year, it referred 11 cases of insider trading and 13 cases of market manipulation.