"In particular, the recent trade tensions between the U.S. and China are worrying concerns", Lee said as ASEAN members gather for the organization's 32nd summit.
"We also noted (North Korea's) reported commitment to denuclearization and its pledge to refrain from further nuclear and missile tests during this period", it added.
ASEAN leaders stressed the need for the implementation of the 2002 Declaration on the Conduct of Parties in the South China Sea (DOC) in its entirety, for there to be mutual trust between parties and the pursuit of a peaceful resolution in the conflict in accordance to global law, including the 1982 United Nations Convention on the Law of the Sea (UNCLOS).
Lee said negotiations for a code of conduct in the South China Sea had started last month between ASEAN and China, and there was hope for an early conclusion.
China, which claims nearly the entire South China Sea and defies a 2016 arbitration award over the disputes in favor of the Philippines, has conducted reclamation work at seven spots, including some ruled to be within Philippine territory, and fortified them with military features.
"We emphasized the importance of non-militarization and self-restraint in the conduct of all activities by claimants and all other states", the statement read.
Myanmar President Win Myint attended the summit.
The situation in Myanmar´s western state of Rakhine, where hundreds of thousands of minority Rohingya Muslims have fled for neighbouring Bangladesh after a military crackdown, is one of the biggest challenges facing the ASEAN group.
The prime minister of Singapore says the Islamic State still poses "very real" threats to the ASEAN region, despite the group's defeat in the Middle East.
The smart cities network, which aims to better connect digital infrastructure and services like e-payments across the region, is Singapore's flagship initiative as the chair of ASEAN this year.
Singaporean Prime Minister Lee Hsien Loong declared at the opening of the plenary session of the 32nd Asean Summit on Saturday that they were opening a new chapter and should adapt to geopolitical changes for the good of their people.
On Saturday Lee said the open and rules-based multilateral trading system, which has backed the growth of ASEAN-member states, has come under pressure as the political mood in many countries has shifted against free trade.
"In particular, the recent trade tensions between the USA and China are worrying concerns", he said.