The vessels involved in two separate fatal collisions in the Singapore Strait had been detected and identified by Singapore's network of maritime sensors, but were not designated as potential security threats, said Defence Minister Ng Eng Hen.

This was rightly so, he told Parliament yesterday.

This meant that they did not require close monitoring by the Republic of Singapore Navy's (RSN) Maritime Security Task Force, which assesses every vessel calling at Singapore or transiting through the Singapore Strait.

Under international navigation regulations, the master and crew of the four vessels are responsible for guiding their ships safely through, Dr Ng added.

"The various parties involved in the collisions will now have to investigate what went wrong and what remedial actions to take if necessary," he said, noting that the Transport Safety Investigation Bureau is conducting an investigation and will make its findings public.

In the case of United States warship USS John S. McCain and Liberian-registered oil tanker Alnic MC, which collided on Aug 21, killing 10 sailors on the warship, both were detected in Singapore waters off Pedra Branca, noted Dr Ng.

The RSN's RSS Gallant, which was patrolling Singapore waters, had also established communications with the foreign warship as part of standard procedures, he added.

As for dredger JBB De Rong 19 and tanker Kartika Segara, both were detected and identified prior to their collision on Sept 13, which took place within Singapore waters off Sisters' Islands, said Dr Ng.

Mr Vikram Nair (Sembawang GRC) had sought an assessment of Singapore's maritime surveillance capabilities following the incidents. He also asked about Singapore's ability to respond to scenarios where vessels in or entering Singapore waters might be used by terrorists.

Dr Ng pointed out that terrorists can target ships in Singapore waters, or attack from sea, as they did in the 2008 Mumbai attack.

Singapore aims to prepare for various scenarios and ensure a coordinated response to threats, he added.

He outlined how the Singapore Maritime Crisis Centre, set up in 2011, brings together the RSN, Police Coast Guard, Maritime and Port Authority (MPA) and relevant agencies to get a comprehensive picture of the maritime situation.

It also taps technology to analyse information, detect suspicious patterns and get agencies to investigate and take action.

In 2015, the centre detected a potential Islamic State in Iraq and Syria sympathiser aboard a tanker calling on Singapore, and the individual was barred from entering the country.

Last year, it received information about a hijacked tanker, and shared it with the Indonesian authorities, as the vessel was in their waters. "This led to the ship's rescue and, in that case, the hijack was not linked to any terror intent," said Dr Ng.

The Maritime Security Task Force feeds information - obtained from monitoring close to 1,000 ships passing through the Singapore Strait each day through a network of sensors - to the crisis centre.

Dr Ng added that if, for example, there was information that a particular ship was carrying illegal cargo or had malignant intent towards Singapore, the task force could board it, or keep it under close surveillance.

Ms Joan Pereira (Tanjong Pagar GRC) asked whether there will be a review of the Vessel Traffic Information System (VTIS), which integrates data from various sources to monitor shipping traffic.

Senior Minister of State for Transport Lam Pin Min said the MPA, which operates the system installed in 2011, regularly enhances it. It was last upgraded early this year.

The VTIS integrates data from radars, the Automatic Identification System, closed-circuit television camera systems and ship databases, among others, to provide an accurate and comprehensive understanding of traffic.

"With the capability to concurrently track more than 10,000 vessels, the VTIS enables MPA to provide timely information and advice to help vessels transit safely through the Singapore Strait, as well as manage traffic within our port waters, where there are some 1,000 vessels at any one time," added Dr Lam.