CANBERRA (Reuters) - US President Barack Obama and Australian Prime Minister Julia Gillard on Wednesday unveiled plans to deepen the US military presence in the Asia-Pacific, with 2,500 US marines operating out of a de facto base in northern Australia. China, already worried the United States is caging it in, immediately questioned whether strengthening military alliances would help the region when economic woes put a premium on cooperation. "With my visit to the region, I am making it clear that the United States is stepping up its commitment to the entire Asia-Pacific region," Obama told a joint news conference with Gillard in Canberra. From next year, US troops and aircraft will operate out of the tropical city of Darwin, only 820 kms (500 miles) from Indonesia, able to respond quickly to any humanitarian and security issues in Southeast Asia, where disputes over sovereignty of the South China Sea are causing rising tensions. "It is appropriate for us to make sure...that the security architecture for the region is updated for the 21st century and this initiative is going to allow us to do that," Obama said. He stressed that it was not an attempt to isolate China which is concerned that Washington is trying to encircle it with bases in Japan and South Korea and now troops in Australia. "The notion that we fear China is mistaken. The notion that we are looking to exclude China is mistaken," he said, adding China was not being excluded from the planned Trans-Pacific Partnership (TTP) on trade. "We welcome a rising, peaceful China." But China's rising power means it must take on greater responsibilities to ensure free trade and security in the region, he added. "It's important for them to play by the rules of the road and, in fact, help underwrite the rules that have allowed so much remarkable economic progress," he said. The US deployment to Australia, the largest since World War Two, will start next year with a company of 200-250 marines in Darwin, the "Pearl Harbour of Australia", Gillard said. More bombs were dropped on Darwin during a surprise Japanese raid than on Pearl Harbour, Hawaii. A total of 2,500 US troops would eventually rotate through the port city. The United States will bring in ships, aircraft and vehicles, as well as increase military training. Asked about the proposed deepening of US-Australian military cooperation, Foreign Ministry spokesman Liu Weimin said China stood for "peaceful development and cooperation". "We also believe that the external policies of countries in the region should develop along these lines," Liu told a regular news briefing in Beijing. Liu added that "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", especially amid a gloomy international economic situation and with each country seeking cooperation. But some Asian nations are likely to welcome the US move as a counterbalance to China's growing military power, especially its expanding maritime operations, and a reassurance that Washington will not scale back its engagement in the region due to a stretched US military budget.