CANBERRA - President Barack Obama landed in Australia on Wednesday where he is expected to announce a deepening of US military presence in the Asia-Pacific, starting with US Marines rotating in and out of the Australian port of Darwin -- a gateway to Asia.

US troops in Darwin, only 820 kms (500 miles) from Indonesia, would be able to react quickly to any security issues in Southeast Asia, eliminating days of sailing from US bases in north Asia or the Pacific.

The move, scheduled to be unveiled during the two day visit, may be seen as further evidence of Washington's attempt to encircle China, with US bases in Japan and Korea and now troops in Australia.

Obama was met by Prime Minister Julia Gillard after stepping off Airforce One and greeted with a 21 gun salute when he arrived at Parliament buildings in the Australian capital.  

The winding down of wars in Afghanistan and Iraq has opened the door to greater US attention to the South China Sea. Obama plans to raise maritime security in the South China Sea at a regional summit on Bali this week.

Obama will make an "anchor speech" outlining the US vision for the Asia-Pacific to the Australian parliament on Thursday before a whistle stop in Darwin before flying to the Indonesian island of Bali for the East Asia summit.

Darwin, nicknamed the "Pearl Harbour of Australia" after a World War Two Japanese raid dropped more bombs on the city than those on Pearl Harbour, will give the US military open access to East Asia sea lanes and the Indian Ocean.

Australia says hosting US troops and the pre-positioning of US supplies in Darwin is not the precursor to a US base, but analysts say rotating more than 2,000 US Marines in and out of the northern port city, and more frequent US naval visits, will give Washington a de facto base.