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 rose in a territorial row.
"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 election victory, described the radar-locking as "unilateral provocative action by the Chinese side".
Abe's comments come a day after Defence Minister Itsunori Onodera announced that 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.
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 protested on Tuesday to Beijing about the incidents and asked for an explanation, but had yet to receive any reply.
Chinese foreign ministry spokeswoman Hua Chunying said Wednesday she was "not aware of the specifics" and referred inquiries to "competent Chinese authorities".
"You can understand in this way: we learned about this incident from the press reports," she told reporters at a regular briefing.
Beijing's defence ministry did not immediately respond to AFP requests for comment.
US State Department spokeswoman Victoria Nuland said Washington was concerned at 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.
Radar is used precisely to determine a target's distance, direction, speed and altitude. Weapon systems linked to the radar can be fired immediately, Japan's government said.
The situation is already tense 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 area and both Beijing and Tokyo have scrambled fighter jets, though there have been no clashes.