YONAGUNI, Japan (Reuters) - Japan began its first military expansion at the western end of its island chain in more than 40 years on Saturday, breaking ground on a radar station on a tropical island off Taiwan.
The move risks angering China, locked in a dispute with Japan over nearby islands which they both claim.
Japanese Defence Minister Itsunori Onodera, who attended a ceremony on Yonaguni island to mark the start of construction, suggested the military presence could be enlarged to other islands in the seas southwest of Japan's main islands.
"This is the first deployment since the U.S. returned Okinawa (1972) and calls for us to be more on guard are growing," Onodera told reporters. "I want to build an operation able to properly defend islands that are part of Japan's territory."
The military radar station on Yonaguni, part of a longstanding plan to improve defence and surveillance, gives Japan a lookout just 150 km (93 miles) from the Japanese-held islands claimed by China.
Building the base could extend Japanese monitoring to the Chinese mainland and track Chinese ships and aircraft circling the disputed crags, called the Senkaku by Japan and the Diaoyu by China.
The 30 sq km (11 sq mile) Yonanguni is home to 1,500 people and known for strong rice liquor, cattle, sugar cane and scuba diving. Prime Minister Shinzo Abe's decision to put troops there shows Japan's concerns about the vulnerability of its thousands of islands and the perceived threat from China.
The new base "should give Japan the ability to expand surveillance to near the Chinese mainland," said Heigo Sato, a professor at Takushoku University and a former researcher at the Defence Ministry's National Institute for Defence Studies.
"It will allow early warning of missiles and supplement the monitoring of Chinese military movements."
Japan does not specify an exact enemy when discussing its defence strategy but it makes no secret it perceives China generally as a threat as it becomes an Asian power that could one day rival Japan's ally in the region, the United States.
Japan, in its National Defence Programme Guidelines issued in December, expressed "great concern" over China's military buildup and "attempts to change the status quo by coercion" in the sea and air.
China's decision last year to establish an air-defence identification zone in the East China Sea, including the skies above the disputed Senkaku/Diaoyu islets, further rattled Tokyo.
Japanese and Chinese navy and coastguard ships have played cat-and-mouse around the uninhabited islands since Japan nationalised the territory in 2012. Japanese warplanes scrambled against Chinese planes a record 415 times in the year through to March, the Defence Ministry said last week.
Tapping concern about China, Abe raised military spending last fiscal year for the first time in 11 years to help bolster Japan's capability to fight for islands with a new marine unit, more longer-range aircraft, amphibious assault vehicles and helicopter carriers. Japan's thousands of islands give it nearly 30,000 km (18,600 miles) of coastline to defend.
Onodera's groundbreaking ceremony on Yonaguni took place s four days before President Barack Obama lands in Tokyo for a summit with Abe, the first state visit by a U.S. president in 18 years.
The United States, which under its security pact with Tokyo has pledged to defend Japanese territory, has warned China about taking any action over the disputed islets, but has not formally recognised Japan's claim of sovereignty over the territory.
Many of the islanders on nearby Yonaguni are looking forward to hosting the radar base and the 100 troops who will man it because of the economic boost it will bring.
Others on the island, however, fear becoming a target should Japan end up in a fight.
"Opinion is split down the middle," Tetsuo Funamichi, the head of the Japan Agricultural Association's local branch, told Reuters. "It's good for the economy if they come, but some people worry that we could be attacked in an emergency."
Onodera was also greeted on Saturday by about 50 protesters who tried to block him from entering the construction site.
"Becoming a target is frightening, they won't talk to us about it, we haven't discussed it," a protestor, who declined to be identified said.
Takenori Komine, who works in an island government office, said it was a risk worth taking if it meant reviving an outpost of Japan that has been in decline since a brief postwar boom.
At that time, U.S.-occupied Yonaguni's proximity to Taiwan made it an entry point into Japan for smuggled food and clothing from Hong Kong. Since the end of World War Two, the island's population has withered by some 90 percent. Average income of about $22,500 a year is a fifth below the national average.
"We are hopeful that the arrival of the young troops will bolster local consumption," Komine said.
(Writing by Tim Kelly; Editing by Angus MacSwan)