TOKYO (AFP) - Japan's prime minister said on Thursday the "window of dialogue" with China must remain open, even as he reiterated his rebuke to Beijing over a naval confrontation on the high seas.

Mr Shinzo Abe said an incident in which a Chinese frigate locked its weapons-targeting radar on a Japanese warship was "extremely regrettable", as tension grows over the sovereignty of islands in the East China Sea.

"But we will not close the window of dialogue. This is most important," Mr Abe said. "I would like China to return to a more open attitude towards our strategic partnership."

Abe Wednesday had described the Chinese action as "dangerous" and "provocative".

Defence Minister Itsunori Onodera told parliament the Chinese radar lock amounted to a "threat of force" but he called for some mechanism to allow defence authorities to communicate with each other.

"We think this is a threat of force, as defined in the UN Charter," Mr Onodera said.

"But what is most important is to prevent incidents like this from recurring in the future," he said. "I also think it is necessary for Japan and China to have a means of consultation on maritime safety issues."

The radar incident, which Japan said happened last month, marked the first time the two nations' navies have locked horns in a dispute that has some commentators warning about a possible armed conflict.

The situation has been tense for months in the East China Sea, where Asia's two largest economies are at loggerheads over the sovereignty of an uninhabited island chain, called the Senkakus in Japan and the Diaoyus in China.

Beijing has repeatedly sent ships to the area since Japan nationalised some islands in the chain in September. The nationalisation move triggered a diplomatic dispute and huge anti-Japan demonstrations across China.

Mr Natsuo Yamaguchi, head of the New Komeito party which is Mr Abe's coalition ally, called for a Japan-China summit by Aug 12, the 35th anniversary of the signing of the Japan-China Peace and Friendship Treaty.

"Avoiding contingencies and pushing for big-picture ties is the duty of the two countries' political leaders," said Mr Yamaguchi, who delivered a letter from Mr Abe to China's incoming president Xi Jinping last month.