Ukraine's prime minister urged Japan's government and private sector on Monday to step up support for the country's reconstruction, promising an "economic miracle" once the almost two-year-old war with Russia ends.

"Since the full-scale invasion started Japan has provided Ukraine with over $10 billion of various support," making it the fourth-biggest donor, Denys Shmygal told a conference of around 300 government and business leaders from Japan and Ukraine.

"Thanks to this funding during this terrible war caused by Russia, millions of Ukrainian people have been able to survive," Shmygal said through an interpreter.

Ukrainian President Volodymyr Zelensky had also been due to address the gathering in Tokyo by video link but this was cancelled, organisers said without elaborating.

"The private sector is the main engine for reconstruction efforts in Ukraine... We would like to work together with Japan on the modernisation and reconstructing of Ukraine. We want you all to be part of the economic miracle for Ukraine," Shmygal said.

"Companies that invest in the Ukrainian economy now will be able to gain benefits and opportunities in the next few years that you would not believe. Ukraine's success is not just our success, it is your success as well."

Japan last year relaxed export controls on finished goods, allowing it to help fill gaps in defence equipment among Ukraine's main suppliers of weapons such as the United States.

Tokyo has also provided to Ukraine what it calls "non-lethal support" such as aircraft detecting systems and mobile power generators.

Last week the World Bank, European Union, United Nations in a joint report estimated Ukraine's reconstruction costs at $486 billion over the next decade, including $15 billion in 2024 alone.

Prime Minister Fumio Kishida on Monday "reaffirmed Japan's commitment to supporting Ukraine and its people in defending its freedom and independence and restoring territorial integrity," according to a joint statement.

He also reiterated a commitment to provide "the necessary long-term assistance to ensure the stability of Ukraine's economy", the statement said.

The two sides signed a number of agreements including on infrastructure rebuilding, demining, agriculture, and governance enhancement.

Japan also agreed to relax visa rules for visiting Ukrainian businesspeople.

The conference comes after Russian forces recaptured the eastern Ukrainian stronghold of Avdiivka and as $60 billion in US military aid remains stalled in Congress.