Prime Minister Lee Hsien Loong placed a bouquet of white flowers and observed a moment of silence at the 9/11 memorial in New York.

Mr Lee, who was accompanied on Monday by his wife Ho Ching and a Singapore delegation, then visited the sprawling underground museum documenting the deadly terrorist attacks on Sept 11, 2001.

The memorial - two large pools set in the original footprint of the World Trade Centre towers, which collapsed after two hijacked planes were crashed into them - was opened in 2011 and has been visited by a host of world leaders.

German Chancellor Angela Merkel laid a wreath beneath the Survivor Tree at the memorial last year, while Pope Francis placed a single white rose on the ledge of one of the pools last September.

Nearly 3,000 people were killed in four coordinated attacks by Al-Qaeda and their names are etched on the panels surrounding the two pools. The memorial also includes the victims of the World Trade Centre bombing in 1993.

The exhibits in the accompanying 110,000 sq ft museum include mangled steel structures from the towers and a brick chiselled out of the house in Abbottabad, Pakistan, where Al-Qaeda leader Osama bin Laden was shot.

"Terrorism has taken many lives, and we must remain resolute in countering it," PM Lee wrote on Facebook later about his visit to the memorial.

"I had just conveyed our deepest condolences to the people of Pakistan and the victims of the brutal terrorist attack on Easter Sunday. It was especially heartrending that many victims were women and children. It is yet another reminder of the need for continued vigilance against terrorism."

Mr Lee wraps up his visit to New York today and heads to Washington DC to attend the Nuclear Security Summit. More than 50 world leaders will attend the meeting starting tomorrow.