By Aaron Sheldrick and Ben Blanchard
TOKYO/BEIJING (Reuters) - Japan has ordered geological survey ships to prepare for possible deployment to the East China Sea after it detected Chinese drilling near the disputed maritime border, a source with direct knowledge of the order told Reuters on Thursday.
A plan by Chinese state-run oil firms to dramatically expand gas drilling in the disputed East China Sea, which threatens to further damage ties between Asia's two biggest economies, was first reported by Reuters on Wednesday.
In a possible sign of brinkmanship, Japanese survey ships, if deployed, would operate "right up to the median line", which is the disputed maritime boundary with China, said the senior Japanese source who declined to be identified due to the sensitivity of the issue.
Tension over the East China Sea has escalated this year, with China and Japan scrambling fighter jets and ordering patrol ships to shadow each other, raising fear that a miscalculation could lead to a broader clash.
The Japan Oil, Gas and Metals National Corporation (JOGMEC) has been ordered to put both its survey ships on standby and to prepare to deploy without any foreign members of staff on board, said the source.
A spokesman at JOGMEC referred inquiries to the government when asked about the possible deployment.
The Ministry of Economy, Trade and Industry, which gave the order, according to the source, also declined to comment. The ministry would normally be in charge of such deployments.
On Thursday, Japan again warned China not to expand gas exploration in the disputed area. China had slowed exploration in the East China Sea but is now rapidly expanding its hunt for gas, a cheaper and cleaner energy to coal and oil imports.
"If the Chinese side is to proceed unilaterally with development in the area over which there are conflicting claims, Japan would never accept it," Japanese Chief Cabinet Secretary Yoshihide Suga told a regular news conference on Thursday.
Japan seemed to be taken by surprise at the news that Chinese oil firms would soon ask Beijing to approve seven new gas fields in the East China Sea under a $5 billion expansion plan.
"We are going to closely watch the Chinese side's reaction to the concerns expressed by our side through diplomatic channels and we will think about our next step," said a spokeswoman in Prime Minister Shinzo Abe's office when asked about any possible deployment.
JOGMEC has two survey ships, the Shigen and the Hakurei. The Shigen is off the coast of Hokkaido in northern Japan and would take about a week to reach the East China Sea, said the source. The Hakurei is in Okinawa, not far from the disputed zone.
Two of China's top newspapers accused Abe on Thursday of dangerous politics that could threaten regional security.
The People's Liberation Army Daily said Abe was trying to play the "China threat", to win votes in upper house of parliament elections on Sunday, with a visit this week to Japan's southern island of Ishigaki, near islets claimed by both China and Japan.
Rival territorial claims between China and Japan over the uninhabited islets and resource-rich waters in the East China Sea, and between China and other neighbors in the South China Sea, are among Asia's biggest security risks.
During the visit to Ishigaki island, Abe repeated Japan's stand that the nearby disputed Senkaku islands, called the Diaoyu by China, are inherent Japanese territory, adding that he has no intention of conceding even one step.
"This kind of 'drinking poison to slake ones thirst' not only threatens regional stability, it gives encouragement to Japan's 'turn to the right'," the Chinese newspaper said.
Abe wants to revise Japan's constitution, drafted by the United States after World War Two, to formalize the country's right to have a military. Critics say his plan could return Japan to a conservative, authoritarian past.
The People's Liberation Army Daily said Abe could not have chosen a worse time to visit Ishigaki, 160 km (100 miles) from the uninhabited islets the two nations contest.
"You cannot criticize a national leader for visiting his country's own territory but in a situation where the dispute over the Diaoyu Islands is continuing and the situation is complex and sensitive, Abe's actions are doubtless extremely dangerous and irresponsible," the paper, the official publication of China's military, said in a commentary.
The ruling Communist Party's official People's Daily warned that China would never allow itself to be trampled on again, a reference to China's bitter memories of Japan's invasion and occupation of the country, before and during World War Two.
In a commentary published under the pen name "Zhong Sheng", or "voice of China", the newspaper said Abe was looking for excuses to re-arm Japan and that the dispute with China was a convenient way of pushing that goal.
"The aim is to create tension and provoke incidents, to push Japan's military development," it said.
On Thursday, three Chinese surveillance vessels sailed into what Japan considers its territorial waters near the isles on what Beijing said was a routine patrol. The Japan Coast Guard said the ships later left its territorial waters.
(Additional reporting by James Topham, Elaine Lies, Kiyoshi Takenaka, Linda Sieg and Antoni Slodkowski in TOKYO; Editing by Michael Perry and Robert Birsel)