The Association of Southeast Asian Nations is hoping to expedite negotiations on a code of conduct with China for the disputed South China Sea but it isn't realistic to expect an agreement within a year, Singapore's defense minister said Wednesday.

China and the 10-member ASEAN bloc adopted a negotiating framework on the code in August and have commenced talks on the code itself over the disputed and busy waterway largely controlled by China but also claimed by some ASEAN states.

"We hope it will be expedited but it's a very, very complex issue," Singapore Defense Minister Ng Eng Hen told reporters after a gathering of ASEAN defense chiefs.

"It's a century's old dispute. Expecting (the code) in one year is just unrealistic," he said.

ASEAN and China have hailed the conclusion of the negotiating framework as a sign of progress.

However, the failure to outline as an initial objective the need to make the code legally binding raised doubts about the effectiveness of the pact.

Signing China up to a legally binding and enforceable code for the waterway has long been a goal for claimant members of ASEAN, some of whom have sparred for years over what they see as China's disregard for their sovereign rights and its blocking of fishermen and energy exploration efforts.

Singapore Foreign Minister Vivian Balakrishnan said Tuesday some of his ASEAN colleagues had expressed concerns about ongoing activities by China in the disputed areas of the South China Sea, including land reclamation.

Malaysia, Taiwan, Brunei, Vietnam and the Philippines all claim some or all of the South China Sea and its myriad shoals, reefs and islands.

Singapore has taken over the role of chairing ASEAN for 2018 and hosted meetings of the group's foreign and defense ministers this week.