The annual leading Asia-Pacific defence and security summit, the Shangri-la Dialogue, gets under way in Singapore on Friday.

It comes amid America's continuing "pivot" or "rebalancing" towards the region and the growing military naval power of China.

The Shangri-La Dialogue is held annually by an independent think tank, the International Institute for Strategic Studies and is attended by defence ministers, permanent heads of ministries and military chiefs of 28 Asia-Pacific states.

Diplomatic correspondent Jonathan Marcus went on board USS Freedom, which is currently stationed in Singapore, to explain why the US is shifting its balance towards the Asia-Pacific region.