Prime Minister Lee Hsien Loong joined leaders from more than 50 countries in the US Capitol for a summit aimed at dealing with the threat of nuclear weapons.
But hours before the meeting took place yesterday, North Korea fired a missile into the sea off its east coast. South Korea's Yonhap news agency said the test appeared to involve a ballistic missile.
It took place just hours after leaders from the United States, South Korea and Japan held a trilateral summit in Washington, at which they vowed to ramp up pressure on North Korea in response to its recent nuclear and missile tests.
US President Barack Obama, South Korea's President Park Geun Hye and Japan's Prime Minister Shinzo Abe recommitted their countries to each other's defence and warned they could take further steps to counter threats from Pyongyang.
Mr Obama on Thursday also hosted leaders attending the nuclear summit to a working dinner at the White House. This will be the final summit in a series of four that was initiated by Mr Obama in 2010.
PM Lee has attended every Nuclear Security Summit, in a sign of Singapore's commitment to global nuclear security.
Chinese President Xi Jinping met US President Barack Obama in Washington on Thursday on the sidelines of the Nuclear Security Summit. Here is what China's Xinhua news agency reported the Chinese leader as saying on the South China Sea situation:
China will not accept any act under the disguise of freedom of navigation that violates its sovereignty and damages its security interests.
Beijing respects and safeguards the freedom of navigation and overflight other countries are entitled to under international law.
China is resolute both in defending its sovereignty and related rights in South China Sea, and in safeguarding peace and stability in the region, and sticks to the principle that the disputes should be settled in a peaceful way by relevant claimants through direct consultations and negotiations.
Beijing hopes that the United States will abide by its commitment to not taking sides on the sovereignty and territorial rows in South China Sea and play a constructive role in maintaining regional peace and stability.
At the end of the meetings, the leaders are expected to release a joint communique outlining steps forward and issue action plans for relevant agencies like the United Nations, International Atomic Energy Agency and Interpol.
This year's summit is taking place against the backdrop of an increasingly belligerent North Korean regime and the heightened threat of nuclear terrorism.
In the aftermath of the deadly terror attacks in Belgium, new concerns were raised that militant group Islamic State in Iraq and Syria was planning attacks on the country's nuclear installations. It emerged after the attacks that there have been troubling breaches at Belgium's two nuclear plants. For instance, two workers quit in 2012 to join the fighting in Syria.
Despite the global threats, experts have low expectations for this year's meeting, especially given the Russian boycott.
Russia - which possesses one of the largest nuclear stockpiles in the world - decided to sit out the summit, citing a lack of mutual cooperation in working out the agenda.
US officials stressed that Moscow's absence would not unduly impact the summit.
"Russia's decision to not participate... we believe is a missed opportunity for Russia above all. They have benefited enormously from cooperation on nuclear security and non-proliferation in the past," said White House Deputy National Security Adviser Ben Rhodes. "All they are doing is isolating themselves in not participating as they have in the past."
PM Lee on Thursday met US Federal Reserve chair Janet Yellen and Trade Representative Michael Froman. They exchanged views on the global economy and on the prospects of the Trans-Pacific Partnership free trade pact.