Beijing fears a policy of encirclement as Barack Obama announces plans to station 250 troops in northern Australia from next year

Strains between China and the United States look set to overshadow a key Asian summit later this week after Barack Obama unveiled plans to station troops in Australia and step up Washington's commitment to the entire Asia-Pacific region.

Amid growing tensions over the oil-rich South China Sea, China's foreign ministry questioned the need for the US to strengthen military alliances, and the state media accused Washington of making a grab for assets.

Beijing's unease has also been heightened by US Secretary of State Hillary Clinton's insistence that the South China Sea issue be included as a topic for discussion at the east Asia summit in Manila, which will be attended by President Obama, President Hu Jintao of China and other regional leaders. China had requested that the sensitive issue be omitted.

Ahead of the summit Obama made a whirlwind 27-hour visit to Australia, where he and Julia Gillard, the prime minister, announced that from next year 250 marines will be stationed in the Northern Territory, rotating in and out of Darwin for around six months at a time. Over the next few years this will build to a force of 2,500 US personnel.

In the last year the Pentagon has also negotiated wider access to military facilities in Vietnam, Singapore and the Philippines, prompting fears in Beijing that it is engaged in a policy of encirclement, as well as becoming involved in the row over waters in the South China Sea, which are claimed by several nations.

"The United States is also trying to get involved in a number of regional maritime disputes, some of which concern China's sovereignty and territorial integrity," a commentary from China's official Xinhua news agency said.

The Global Times, a Communist party paper, said the Philippines, aided and abetted by the US, was intent on "grabbing resources from Chinese water".

Obama insisted, however, that the US was not attempting to isolate China. "We welcome a rising, peaceful China," he said. But he claimed that there will be times when Beijing does not play by the rules and the US will send a "clear message that they need to be on track in accepting the rules and responsibilities that come with being a world power".

The troop announcement comes on the 60th anniversary of the Australia-New-Zealand-United-States (Anzus) defence treaty, which binds the three countries to assist each other if attacked.

As well as new troops, the US air force will also have greater access to Australian air bases and there will be more frequent flights of US aircraft in and out of northern Australia.

"Our alliance is going to be indispensable to our shared future and the security we need and the prosperity we seek, not only here but around the world," Obama said.

But it was questioned by China's foreign ministry spokesman Liu Weimin. "Whether strengthening and expanding a military alliance is in the common interests of the region's countries and the international community is worthy of discussion," he said.

China, he said, stood for peaceful development and co-operation. "We also believe that the external policies of countries in the region should develop along these lines," Liu told a news briefing in Beijing.

Dr James Curran, senior lecturer in history at Sydney University, said this new deployment would help to assuage some of the US allies in the region about China's increasing power.

"This is a very strong statement that the US is in the region to stay," he said.