Several Asian nations are seeking to bolster informal alliances among themselves, regional diplomats and officials said, unsettled by growing fears that the United States could not be relied on to maintain a buffer against China's assertiveness.

Countries including Australia, Japan, India and Vietnam are quietly stepping up discussions and cooperation, although taking care they do not upset Beijing, the diplomats said. No one was yet talking about a formal alliance.

Inaugurating the weekend Shangri-La Dialogue, the region's premier security forum, Australian Prime Minister Malcolm Turnbull said: "In this brave new world we cannot rely on great powers to safeguard our interests.