"We are happy that the Americans have taken a stand against ISIS. This is a threat to people all over the world," he said on Thursday during a visit to Hong Kong.

But the situation is a difficult and complicated one, Lee added.

Singaporeans, Malaysians and Indonesians were among the 12,000 foreign fighters who have joined the ISIS ranks.

"How can we support the American-led effort? That's something we can discuss. We have not reached a point of making a decision yet," the Prime Minister said.

"Even how the Americans are going to pursue this and what they're going to do, they haven't quite decided yet," he was quoted as saying.

"They're not going to put troops on the ground. It's not one of those situations where you can say, 'Those are bad guys, let's knock them out'."

"You're talking about 30-something thousand fighters, they command a very substantial piece of land, they have resources… they've got oil. And they were also fighting the government in Syria, which, (until) very recently... was seen as bad guys. (They’re) also fighting against the Iranians.”

"In the Middle East, things are never so simple and you can't come riding in on a white horse, knock out the bad guys and tomorrow, peace breaks out," Lee said.

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