TOKYO (AFP) - The radar lock that a Chinese frigate put on a Japanese warship was "dangerous", Japan's Prime Minister Shinzo Abe said on Wednesday, as tensions in a territorial row heightened further.

"It was a dangerous act that could have led to an unpredictable situation," Mr Abe told parliament. "It is extremely regrettable. We strongly ask for their self-restraint in order to avoid an unnecessary escalation."

Mr Abe's comments come a day after his defence minister 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 the US was "concerned".

"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.