The use of chemical weapons cannot go unpunished, said Prime Minister Lee Hsien Loong, noting that this was a breach of the rule of international law.
As a small state, Singapore took seriously the need to uphold the rule of law, he added.
Besides, being a compact city-state, Singapore would have a "severe problem" if chemical weapons were ever to be deployed.
This was why Singapore had thought out its position carefully and concluded that the use of such weapons could not be condoned, he said.
He noted that while it was still unclear exactly who had deployed the chemical weapons in Syria, he was sure this would eventually be established.
"Whoever did this, really, it should not go unpunished," he said, adding that just how to do so was something that had to be worked out.
Ideally, any punitive response should be taken under the auspices of the United Nations, which clearly had the authority to act, he said, since chemical weapons have been outlawed by international treaties.
But, he acknowledged, this was not always possible, and if so, some other ways might need to be found to "punish the perpetrators in a way that is consistent with international law".
Mr Lee was speaking to the Singapore media on Friday, at the close of the Group of 20 (G-20) summit held in St Petersburg.
The two-day summit saw leaders from G-20 countries and six non-G-20 guests, including Singapore, discussing ways to boost growth and jobs in the global economy, in the Konstantinovsky Palace, which was built in the early 18th century as a summer home for Peter the Great.
But the shadow of Syria loomed large over their discussions, with a stand-off between the United States - which is pushing for a global response to the alleged use of chemical weapons by Syria - and Russia, a long-time Syrian backer which rejects any US-led military strike.
Noting that Syria was not on the official agenda, Mr Lee said that it was one of those "unexpected storms" that arose and had to be discussed. Countries expressed their views on the issue "trenchantly", noted Mr Lee.
The discussions went on late into the night, with a dinner discussion which focused on Syria running well past midnight, with no clear consensus emerging.
US President Barack Obama had said he did not expect to settle the matter at the meeting, but put forth his case for action forcefully.
Said Mr Lee: "Those who were of like mind with the Americans, like the UK, also expressed their views strongly. Those who have different views, and there were many, also expressed their positions. I don't think we came to a meeting of minds, but it crystallised the different perspectives on this issue."
Wrapping up the summit, Mr Putin disclosed that he had held an unexpected meeting with Mr Obama, which both sides said was "constructive". But the Russian leader remained combative and insisted that most of the leaders present supported his position.
Attention now turns to the US, where Congressional leaders are expected to vote on whether to back the use of force next week. firstname.lastname@example.org
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