NAYPYITAW (Myanmar) - The United States' long-standing presence in Southeast Asia has underpinned the region's growth and prosperity, Prime Minister Lee Hsien Loong said on Thursday.

But he also acknowledged that the Asean-US relationship will face many pressing challenges ahead, and that both sides must consider how to address them together.

Speaking at the second Asean-US summit on the sidelines of the 25th Asean Summit, Mr Lee thanked President Barack Obama for keeping Asean at the forefront of US policy, and said both Asia and Asean are increasingly important to the US' security and economic interests.

"As Secretary (of State John) Kerry has said (at the Informal Asean-US Ministerial Meeting in September), Asean centrality was essential to upholding a rules-based system in the Asia-Pacific and was the best way to ensure all countries, big and small, had a voice," Mr Lee said, adding that Washington's support for Asean centrality was a crucial component of their partnership.

It was also important to bear in mind that the Asia-Pacific is a diverse region where "it will be difficult to rationalise architecture into one arrangement", he added, referring to the network of sometimes overlapping partnerships.

In his remarks, Mr Lee said Asean and the US should enhance economic cooperation to boost regional growth and create opportunities. He thanked Mr Obama for organising a Trans-Pacific Partnership leaders' meeting in Beijing on Monday, when leaders attended the Apec Summit there.

Asean is expected to overtake Japan to become the fourth largest single market in the world - with combined GDP of US$10 trillion - by 2030.

Firm US leadership will also be required to counter fast-breaking crises like the Ebola outbreak and the Islamic State of Iraq and Syria (ISIS), Mr Lee said, adding that Singapore is ready to support where it can.

Singapore has contributed an aid package to the World Health Organization's Ebola response efforts, and has joined more than 30 countries in a multinational coalition against ISIS.

On the South China Sea territorial dispute, Mr Lee said that Singapore welcomed American support for progress on a Code of Conduct between Asean and China that will lay down the ground rules for engagement in the area.

"Singapore stands ready to play our part to strengthen Asean-US relations," he said.