* China unveils another double-digit hike to its defence budget
* Officials in Japan and Taiwan express concern
* Much defence spending likely takes place outside the budget
* US official questions use of powerful Chinese military (Adds U.S. reaction)
By Michael Martina and Greg Torode
BEIJING/HONG KONG, March 5 (Reuters) - China announced its biggest rise in military spending in three years on Wednesday, a strong signal from President Xi Jinping that Beijing is not about to back away from its growing assertiveness in Asia, especially in disputed waters.
The government said it would increase the defence budget by 12.2 percent this year to 808.23 billion yuan ($131.57 billion), as China seeks to develop more high-tech weapons and to beef up coastal and air defences.
The increase follows a nearly unbroken run of double-digit hikes in the Chinese defence budget, second only to the United States in size, for the past two decades.
"This is worrying news for China's neighbours, particularly for Japan," said Rory Medcalf, a regional security analyst at the independent Lowy Institute in Sydney.
Those who thought Xi might prefer to concentrate on domestic development over military expansion in a slowing economy had "underestimated the Chinese determination to shape its strategic environment", he added.
The 2014 defence budget is the first for Xi, a so-called princeling - or a son of a late Communist Party elder - and the increase in spending appears to reflect his desire to build what he calls a strong, rejuvenated China.
Xi also recently urged China's military leadership to work faster to get the country's sole aircraft carrier combat-ready. The spending jump is the biggest since a rise of 12.7 percent in 2011.
Within hours of the announcement, officials in Japan and Taiwan expressed disquiet over the absence of any details on how Beijing will spend the money, concerns long echoed in Washington.
China and Japan, a key U.S. ally in the region, are increasingly locking horns over uninhabited rocky islands each claims in the East China Sea.
China's military is not made up of "boy scouts with spears", Chinese Foreign Ministry spokesman Qin Gang told a briefing, in response to criticism from Japan.
"Some foreigners always expect China to be a baby scout," Qin said. "In that way, how can we safeguard national security and world peace?"
Beijing also claims 90 percent of the 3.5 million sq km (1.35 million sq mile) South China Sea, which is believed to be rich in oil and gas. The Philippines, Vietnam, Malaysia, Brunei and Taiwan all claim parts of those waters.
Speaking at the opening of China's annual session of parliament, Premier Li Keqiang said the government would "strengthen research on national defence and the development of new- and high-technology weapons and equipment," and "enhance border, coastal and air defences".
"We will comprehensively enhance the revolutionary nature of the Chinese armed forces, further modernise them and upgrade their performance, and continue to raise their deterrence and combat capabilities in the information age," Li told the largely rubber-stamp National People's Congress. He gave no specific details.
China's military spending has allowed Beijing to create a modern force that is projecting power not only across the disputed waters of the East and South China Seas, but further into the western Pacific and Indian Oceans.
Much of China's military spending likely takes place outside the budget, however, and many experts estimate real outlays are closer to $200 billion. For the same of comparison, the U.S. Defense Department's base budget for the 2014 fiscal year is $526.8 billion.
China's military budget spike comes as Asia reacts nervously to a string of recent moves by China to assert its sovereignty in disputed territory, expand its military reach and challenge the traditional dominance of U.S. forces in the region.
Chinese fighters and surveillance planes now routinely patrol a controversial new air defence identification zone that covers disputed Japanese-administered islands in the East China Sea. Beijing's aircraft carrier, meanwhile, went on its first exercises in the South China Sea late last year.
At a time when Washington has stepped up its military presence in the region as part of a strategic "pivot" toward Asia, China is building new submarines, surface ships and anti-ship ballistic missiles, and has tested emerging technology aimed at destroying missiles in mid-air.
Nevertheless, experts say it could be decades before China's military is a match for America's armed forces.
CHINA URGED TO BE TRANSPARENT
Admiral Samuel Locklear, head of the U.S. Pacific Command, told U.S. lawmakers on Wednesday that a strong China had the potential to advance global security.
"What's frustrating, though ... is what's kind of happening in their own backyard as it relates to their relations with some of our allies and our partners," Locklear said, mentioning the South China Sea, as one example.
"The question is: 'Is (China's military) transparent? What is it used for?'" he asked.
The United States last month said it was concerned that China's maritime claims in the South China Sea were an effort to gain creeping control of oceans in the Asia-Pacific region.
Japan's chief cabinet secretary, Yoshihide Suga, said China's lack of clarity in its defence policy and spending was a global concern.
David Lo, a spokesman for Taiwan's Defence Ministry, said that while noting the "substantial" spending increase was needed to modernise China's military, much remained hidden.
"The transparency of China's defence budget has always been questionable, as it is widely seen there are a massive amount of military items hidden," he said.
China has repeatedly said that the world has nothing to fear from its military spending, which it says is needed for legitimate defensive purposes and to modernise equipment.
Fu Ying, a spokeswoman for the parliamentary session, reiterated that policy on Tuesday, saying China was seeking peace through "strength".
China would "respond effectively" to provocations by those ready to sabotage regional security and order, she added.
($1 = 6.1430 Chinese yuan) (Additional reporting by Li Hui and Ben Blanchard in Beijing, John Ruwitch in Shanghai, Nobuhiro Kubo in Tokyo, Faith Hung in Taipei and Missy Ryan and David Brunnstrom in Washington; writing by Greg Torode, editing by Dean Yates and G Crosse)