SINGAPORE, April 1 (Reuters) - Singapore has designated 17 transport companies as critical firms subject to greater regulatory scrutiny under a new law effective on Tuesday, which authorities have said aims to guard against extreme scenarios involving "malicious actors".

The Transport Sector (Critical Firms) Act would give the government tighter control over major Singaporean companies in the air, land and sea transport sectors, with approval required for key leadership changes and when an entity gains control of more than 5% of a firm.

The companies include Singapore-listed SIA , ground services handling firm SATS LTD. and bus and rail operator SBS Transit.

The companies must also inform authorities of events like lawsuits and insolvency that impair their ability to provide essential services, according to the law published in the government gazette.

Unlisted companies also designated as critical include Changi Airport Group, public transport operator SMRT Corporation, Jurong Port and PSA Marine.

A Singapore Airlines spokesperson said the company notes its inclusion and "will be in compliance with the act".

SATS, SIA Engineering Company and SBS Transit did not immediately respond to requests for comment.

In remarks to parliament in May last year, Transport Minister Chee Hong Tat said the law was designed to guard against "extreme scenarios" in future of malicious actors gaining control and adversely influencing transport companies and jeopardising services.

Companies subject to the law were strategically important firms whose services were not readily replaceable due to their significant market shares or expertise, he said at the time.