Japanese Foreign Minister Yoko Kamikawa visited Ukraine on Sunday to meet with her counterpart Dmytro Kuleba to show Tokyo's continued support, a ministry official said.

The unannounced visit was a change to her original two-week tour plan of visiting Poland, Finland, Sweden, the Netherlands, the United States, Canada, Germany, and Turkey from Friday.

Japan has shown support for Ukraine since Russia's invasion, expanding sanctions against Moscow including export bans and asset freezes.

Kamikawa was due to travel to Kyiv from Warsaw on Sunday, the foreign ministry said in a statement.

A ministry official confirmed to AFP that the minister had "arrived somewhere in Ukraine".

During her visit, Kamikawa "will once again reiterate to the Ukrainian side that Japan's consistent policy of standing with and supporting Ukraine remains unchanged," the ministry statement said.

She will brief the Ukrainian side on the details of assistance projects Japan will offer to Kyiv, it said.

Japan plans to host a conference to promote Ukraine's economic reconstruction in Tokyo in February, with Ukrainian Prime Minister Denys Shmyhal planning to attend.

Kamikawa will consult with the Ukraine side about details of the conference and will attend "the handover ceremony of large-scale power-related equipment for winterization assistance," it said.

During her visit, Kamikawa "is to demonstrate Japan's determination to uphold the international order based on the 'rule of law' from the perspective that unilateral changes to the status quo by force, such as Russia's aggression against Ukraine, cannot be accepted," it said.

Japanese Prime Minister Fumio Kishida has repeatedly stressed the importance of rules-based international order as Tokyo is concerned about its own territorial disputes with its neighbors including an increasingly assertive China.

At the Group of Seven summit in Japan in May last year, the bloc's leaders agreed to "starve Russia of G7 technology, industrial equipment and services that support its war machine".