SINGAPORE - The separatist groups in South-east Asia pose a "clear and present" danger to Singapore, said Prime Minister Lee Hsien Loong, as Singaporeans have been drawn to fight for their causes.

Mr Lee also said that if caught, they will be allowed to return home. "But we will make sure he has not brought back any dangerous ideas and is not likely to do any harm.''

He made these points in an interview with Australia's ABC Radio National on Saturday (June 10) when he was asked to assess the terror threat in Singapore.

"We worry about extremist terrorism a lot," he said.

"The Middle East seems a long way away but it is not. This is a problem which is amidst us."

Singaporeans, though few, are among the Malaysians, Indonesians and others from South-east Asia that have gone to the Middle East to join the fight.

Some others have gone to Southern Philippines, Mr Lee noted, and linked up with separatist groups there. They include the Maute, a group that took over Marawi city last month, and the militant Abu Sayyaf group.

Said the Prime Minister: "It is a clear and present danger."

The Philippines had said Singaporeans were among the foreign fighters in the Maute group that attacked Marawi, a largely-Muslim city with more than 200,000 people.

It did not give further details on the Singaporeans or their roles in the Maute group, which has pledged allegiance to terror group Islamic State in Iraq and Syria.

Separately, Singapore's Home Affairs Ministry confirmed that a Singaporean man, who has been in southern Philippines since the 1990s, has been implicated in terrorism activities there.

But the ministry said there is no indication the man, who was not identified, is involved in the ongoing insurgency.

Mr Lee also said a potential attack in Singapore "is a matter of when and not whether."

"We assume that one day, something will happen in Singapore and we are doing our best to prepare ourselves."

The measures include preparing the population psychologically and making sure Singapore's multi-racial society does not come under "catastrophic stress", he said.

"The risk is not just the casualties from the physical attack but the psychological damage done to the trust and confidence between people of different races and religions in Singapore, particularly between the Muslims and the non-Muslims," he added.

Mr Lee's comments follows a commitment made by Defence Minister Dr Ng Eng Hen last week at the Shangri-La Dialogue, an annual regional security forum.

Dr Ng said Singapore is ready to assist in joint patrols or other counter-terrorism initiatives by Indonesia, Malaysia and the Philippines.

It is in Singapore's interests to join these efforts because if terrorists gain a foothold in the region, they can spread their violence to other Asean cities, including Singapore, he added.