BEIJING - China on Thursday once again urged Japan to "immediately and unconditionally" release Chinese nationals illegally detained by the latter over the Diaoyu Islands issue.

Chinese Vice Foreign Minister Zhang Zhijun made the requirement and lodged solemn representations with Japan during his phone call to his Japanese counterpart Sasae Kenichiro.

He asked the Japanese side to ensure the Chinese nationals' personal safety, dignity and fundamental rights.

A working group from the Chinese Embassy and Consulates in Japan has arrived in Okinawa, visited the detained Chinese nationals and lodged representations with the Japanese side on relevant matters.

The Hong Kong Special Administrative Region Government and its representative office in Tokyo also sent officials to join in the representations.

Okinawa police arrested 14 Chinese nationals on suspicions of "illegal entry" after they arrived at the Diaoyu Islands via a Hong Kong fishing vessel on Wednesday to assert China's territorial claim to the islands.

On Wednesday, China's Vice Foreign Minister Fu Ying summoned Japanese Ambassador to China Uichiro Niwa and made a phone call to her Japanese counterpart Tsuyoshi Yamaguchi on the matter.

Fu reiterated China's sovereignty over the Diaoyu Islands and the affiliated islets, and demanded the Japanese side guarantee the safety of the 14 Chinese nationals and free them immediately and without any conditions.

Also on Wednesday, Hong Kong's Chief Executive CY Leung said he was monitoring developments involving the detained citizens, calling on the Japanese government to ensure their safety.

Shen Danyang, spokesman of the Ministry of Commerce, said Thursday that China was greatly concerned about the issue of the illegal detention of the Chinese nationals, urging Japan to properly handle the situation.