China has said it will use unmanned drones to carry out marine surveillance as it tries to increase its presence around uninhabited East China Sea islands at the centre of a dispute with Japan.

Protests against Japan for its control of disputed islands spread across more than a dozen cities in China and turned violent at times today, with protesters hurling rocks at the Japanese Embassy and clashing with Chinese paramilitary police before order was restored.

The Japanese government has agreed to buy several privately owned islands in the East China Sea that are controlled by Japan but also claimed by China, media reports said.

Japan today called on South Korea to end its "illegal occupation" of tiny islets in the strongest language so far in a dispute that has sent the two countries' relations to the lowest levels in years.

Several Japanese nationalists landed yesterday on a rocky island in the East China Sea at the heart of a territorial row with Beijing, sparking protests in several Chinese cities and a diplomatic rebuke from Beijing.

The Chinese and Japanese prime ministers held an impromptu meeting in a hallway at a conference in Europe, in the highest-level contact between the countries since a bitter territorial dispute erupted a month ago, both governments confirmed Tuesday.

Despite a thriving economic partnership, political ties between Japan and China are at their lowest ebb in years.