"Financial support is very necessary, as well as military support," he said, acknowledging growing acceptance that the military conflict could drag on.
"We should be ready that this war will last longer than we expected,” he said, noting that G7 partners were no longer pushing Ukraine to accept an end to war - as they had last year - but were now signaling their support for a longer conflict.
Marchenko said there was also growing openness among donor countries to explore using Russia's frozen assets to pay for Ukraine's reconstruction than even six months ago.
"Our partners are thinking about the possibility of using Russian assets as a necessary tool to support Ukraine.
Marchenko said U.S. Treasury officials had told him the United States had a low amount of Russian assets, but the issue was of greater concern in Japan, Switzerland and EU countries.