TOKYO - Japan has withheld dues for the UN cultural agency, saying it wants to make sure the organization functions properly, a gesture seen by local media as a protest against the registration for world heritage status of China's documents relating to the 1937 Rape of Nanking massacre.

Foreign Minister Fumio Kishida, asked by a reporter, confirmed that Japan withheld UNESCO dues of nearly 4 billion yen ($40 million) for this year. He refused to say whether it was to protest UNESCO's listing last year of Chinese Rape of Nanking documents as a memory of the world.

Kishida said the decision is based on "comprehensive" observations. Japan disputes China's historical views on the 1937 massacre and has criticized the UNESCO decision.

A Foreign Ministry official later said that Japan is watching to see if UNESCO is living up to its founding purpose of promoting peace among member nations through education, and will consider the timing of its payment while examining whether the UN body is operating appropriately.

There is no deadline for a member nation to pay its annual contribution. Nonpayment could lead to a member nation losing its voting right at the body's general meeting, Kishida said.

Japan provides 9.7 per cent of UNESCO budget, the body's second-largest donor after the U.S., which is at 22 per cent. Washington has suspended its dues since 2011, when Palestine began participating in UNESCO.

Differences in views on wartime history have long strained relations between Japan and China.

In 2014, China submitted documents on the 1937 massacre of Chinese citizens by the Japanese military in what became known as the Rape of Nanking, which Japan has largely played down. The documents were registered on the heritage list last October.

China says up to 300,000 people were killed, while Japanese nationalists have largely played down or denied the incident. Japan and some scholars say the number was much smaller.

Japan has criticized UNESCO's "unilateral" registration system, which it says failed to give Tokyo access to the documents for verification, adding that the process lacked fairness and transparency.