Singapore will do its best to keep ties between Asean and China "on an even keel" to expand cooperation and narrow differences between both sides, said Ambassador-at-large Tommy Koh yesterday.

But Singapore, which is currently coordinating Asean-China ties, can succeed in its efforts only if Beijing "acts with wisdom and self-restraint towards Asean, bearing in mind the asymmetry in size and power", Professor Koh added.

The senior diplomat, speaking at the launch of a book on 50 years of relations between Singapore and China, said the Republic's ties with Beijing are inevitably affected by China's relations with Asean.

Beijing's ties with countries in the region were rocky from 1949 to 1979, when it supported communist insurgencies, but a policy shift under Deng Xiaoping and a charm offensive by Chinese leaders put ties on an upward path for the next 30 years. Asean-China ties are now the group's most substantive ones.

But these ties have "unfortunately been on a downward trajectory since 2009", Prof Koh said.

That year, China submitted to the United Nations a map of the South China Sea with nine dashed lines, outlining its claims in those waters. Four Asean members - Brunei, Malaysia, the Philippines and Vietnam - are among those disputing these claims.

The map has "caused a lot of misunderstanding" among Asean nations as China has not adequately explained the legal significance of the lines or the precise nature of its claims, said Prof Koh.

Asean does not take a position on the merits of differing claims, but has a common position on principles such as the peaceful settlement of disputes, in line with international law, and not using force, he added.

"It is, therefore, not possible for Asean to yield to China's demand that Asean should not take a group position on the South China Sea. The South China Sea is part of our neighbourhood, and Asean wants peace and the rule of law to prevail in the South China Sea," he said.

"Asean also wants good relations with China," he added.

China and Asean should not allow the differences on the South China Sea to overshadow their "overall positive agenda" and prevent cooperation in many areas where their interests come together, he said.

The book by World Scientific Publishing, called Singapore-China Relations: 50 Years, costs $58 (hardcover) and $26 (paperback).