HANOI - Singapore is on its guard in the light of the heightened global terror threat, as there is no foolproof protection against an attack like the one London suffered on Wednesday, Prime Minister Lee Hsien Loong said on Friday (March 24).

"It has happened elsewhere, nobody can say it will never happen in Singapore," he said.

"If it does happen, we need to have that fortitude and resilience, and unity, to say: This is bad, let's find out what was the background, let's do what we need to do, but at the same time, carry on with our lives. And tomorrow is another day, Singapore survives."

Mr Lee was speaking to Singapore reporters at the end of his four-day visit to Vietnam.

He was asked whether such an attack could happen in Singapore, and what he would say to those who are concerned it could happen anytime, anywhere.

"It's something we have to be conscious about, these things can happen. And they have happened in many other cities, it could happen to Singapore," he said.

"It is one person, we don't know the circumstances exactly: He had a criminal record, he may have been a criminal now adopting the ISIS flag, he could have been converted and adopted the ideology of extremism, jihadism. We are not sure yet," he added.

"But in our modern, civilised world, we cannot be fortified every single moment of our lives. We have to go on the basis that we are working together. There's peace, we trust one another, we work with one another," he said.

Mr Lee added: "We are on our guard, but at the same time, we know that there is no 100 per cent foolproof protection, and it can happen elsewhere."

On Wednesday, a lone attacker launched an assault outside the Houses of Parliament in London, killing three people and injuring 40 others before he was shot dead. On Thursday evening, another seriously injured victim died.

The British-born attacker had mowed down pedestrians with his car before crashing it into a railing outside the compound, then tried to enter the grounds and stabbed a policeman.

On Thursday, Mr Lee joined other Singapore leaders in expressing shock, sadness and solidarity with Britain over the attack.