The first contingent of 200 United States Marines has arrived in Darwin.

The troops are there on a six-month rotational basis and will take part in training exercises with the Australian Defence Force.

The two countries are boosting defence ties, with the US eventually deploying a 2,500-strong force in northern Australia by 2017.

The move has irked Beijing but US and Australian leaders have stressed it is not an attempt to contain China.

Welcoming the troops, Australia's Defence Minister Stephen Smith said the decision to host them was a response to a changing global balance.

"The world needs to essentially come to grips with the rise of China, the rise of India, the move of strategic and political and economic influence to our part of the world," he said.

In a joint statement, Mr Smith, Prime Minister Julia Gillard and Northern Territory Chief Minister Paul Henderson said the deployment was ''an evolution of existing exercises and activities'' that the defence forces of both countries are already engaged in.

Analysis This is the first time the United States has committed to a permanent rotation of its forces through Australia. In military terms the deployment is modest, but the presence of US troops in yet another Pacific location is being seen by many as a sign of Washington's realignment of its strategic forces to meet the growing presence of China. With US bases already in South Korea, Japan, Guam and elsewhere, the Chinese have questioned the need for more American forces across the Pacific and say they feel encircled. The Australian defence minister has been more explicit in explaining why more American troops are needed, saying the world has to come to grips with the rise of China and India and the move of strategic political and economic influence to the region.

This, they added, was the latest chapter in a 60-year security relationship and would increase the ability of both forces to engage in joint training and operations.

"There are no US military bases in Australia, and this will not change," they added.

'Long-term' prospects

Last week, Australia played down reports that it planned to allow a US air base on islands in the Indian Ocean.

A US newspaper reported the plans, saying this would be a strategic point to fly spy planes over the South China Sea.

TheWashington Post reportsaid that the two countries were planning ''a major expansion of military ties'', including plans for drone flights from the Cocos Islands - a pair of coral atolls in the Indian Ocean north-west of Australia.

But Mr Smith, the defence minister, described the plans as ''long-term'' and ''down the track''.

The troop deployment in northern Australia was announced by Ms Gillard in November 2011 when US President Barack Obama visited the country.

It comes amid growing tensions over maritime disputes in the South China Sea.

China, the Philippines, Vietnam, Malaysia, Brunei and Taiwan have overlapping territorial claims in the area believed to contain rich reserves of oil and natural gas, as well as key shipping lanes.

Canberra and Washington have been long-time allies, but China is Australia's biggest trading partner.