A Japanese city has registered a same-sex couple as partners under the same address, an unusual step hailed by the men as "groundbreaking" in a country without marriage equality.

In Japan, everyone registers their address with local authorities, and Keita Matsuura, 38, said he used to be listed separately from his partner Yutaro Fujiyama, despite living together.

When the pair moved to Omura, in the southern region of Nagasaki, they told city authorities that they wanted to be registered together, like a married couple would be.