The radar-lock that a Chinese frigate put on a Japanese warship was "dangerous" and "provocative", Japan's Prime Minister Shinzo Abe said Wednesday, as tensions in a territorial row ratcheted up.
"It was a dangerous act that could have led to an unpredictable situation," Abe told parliament. "It is extremely regrettable. We strongly ask for their self-restraint in order to avoid an unnecessary escalation."
The hawkish prime minister, who took office late December following a landslide win in elections, described the radar-locking as "unilateral provocative action by the Chinese side".
Abe's comments come a day after Defence Minister Itsunori Onodera announced weapon-targeting radar had been directed at the Japanese vessel in international waters of the East China Sea last week.
The move marks the first time the two nations' navies have locked horns in a dispute that has some commentators warning about a possible armed conflict.
US State Department spokeswoman Victoria Nuland said Washington was "concerned" over the incident.
"With regard to the reports of this particular lock-on incident, actions such as this escalate tensions and increase the risk of an incident or a miscalculation, and they could undermine peace, stability and economic growth in this vital region," she said.
Onodera said a Japanese military helicopter was also locked with a similar radar on January 19.
Deputy Chief Cabinet Secretary Katsunobu Kato told a news conference that Tokyo lodged a protest against Beijing over the radar-locking on Tuesday and asked for an explanation, but was yet to receive any reply.
Radar is used to precisely determine a target's distance, direction, speed and altitude. Weapon systems linked to the radar can be fired immediately, Japan's government said.
The move is a ratcheting-up of an already tense situation in the East China Sea, where Asia's two largest economies are at loggerheads over the sovereignty of an uninhabited island chain.
On Tuesday Tokyo summoned China's envoy in protest at the presence a day earlier of Chinese government -- but not military -- ships in the waters around the Tokyo-controlled Senkakus, which Beijing claims as the Diaoyus.
Beijing has repeatedly sent ships to the area since Japan nationalised some islands in the chain in September. The move triggered a diplomatic dispute and huge anti-Japan demonstrations across China.
Beijing has also sent air patrols to the archipelago and recently both Beijing and Tokyo have scrambled fighter jets, though there have been no clashes.