China and Japan engaged in a fresh round of invective over military movements near a disputed group of uninhabited islands, fuelling tension that for months has bedevilled relations between the Asian powers.
An increasingly muscular China has been repeatedly at odds with others in the region over rival claims to small clusters of islands, most recently with fellow economic giant Japan which accused a Chinese navy vessel of locking radar normally used to aim weapons on a Japanese naval ship in the East China Sea.
China's Defence Ministry rejected Japan's complaint about the radar, its first comment on the January 30 incident. It said Japan's intrusive tracking of Chinese vessels was the "root cause" of the renewed tension.
A Japanese official dismissed the Chinese explanation for incident saying China's actions could be dangerous in the waters around the islets, known as Diaoyu in China and Senkaku in Japan, believed to be rich in oil and gas.
Japanese Prime Minister Shinzo Abe led his conservative party to a landslide election victory in December, promising to beef up the military and stand tough in territorial disputes.
On Thursday, another border problem was brought into focus when Japan said two Russian fighter jets briefly entered its air space near long-disputed northern islands, prompting Japan to scramble combat fighters. Russia denied the accusation.
The commander of U.S. forces in the Asia-Pacific said the squabble between Japan and China underlined the pressing need for rules to prevent such incidents turning into serious conflict.
"What we need in the South China Sea is a mechanism that prevents us turning our diplomacy over to young majors and young (naval) commanders ... to make decisions at sea that cause a problem (that escalates) into a military conflict that we might not be able to control," Admiral Samuel Locklear told a conference in the Indonesian capital.
China is in dispute with several Southeast Asian countries including the Philippines and Vietnam over parts of the South China Sea, which is potentially rich in natural resources.
Locklear said governments and their leaders had to understand the potential for things to get out of hand.
"In this case, I think that point has been made pretty clear," he said in reference to international reaction to the dispute between China and Japan.
China's Defence Ministry, in a faxed statement late on Thursday, said Japan's complaints did not "match the facts". The Chinese ship's radar, it said, had maintained regular alerting operations and the ship "did not use fire control radar".
The ministry said the Chinese ship was tracked by a Japanese destroyer during routine training exercises. Fire control radar pinpoints the location of a target for missiles or shells and its use can be considered a step short of actual firing.
Japan, the ministry said, had "made irresponsible remarks that hyped up a so-called China threat, recklessly created tension and misled international public opinion".
"Japanese warships and airplanes have often conducted long periods of close-range tracking and surveillance of China's naval ships and airplanes," the Chinese Defence Ministry said.
"This is the root cause of air and maritime security issues between China and Japan."
In Tokyo, Japan's Chief Cabinet Secretary Yoshihide Suga told a news conference Japan could not accept China's explanation and Japan's accusation came after careful analysis.
"We urge China to take sincere measures to prevent dangerous actions which could cause a contingency situation," Suga said.
Japanese Defence Minister Itsunori Onodera said this week that the radar incident could have become very dangerous very quickly, and it could have been seen as a threat of military force under U.N. rules.
Hopes had been rising recently for an easing of the tension, which was sparked, in part, by Japan's nationalization of three of the privately owned islets last September.
Fears that encounters between aircraft and ships could bring an unintended clash have given impetus to efforts to improve links, including a possible summit between Abe and Chinese leader Xi Jinping, who takes over as head of state in March.