U.S. President Joe Biden will use the remaining four months of his term "to put Ukraine in the best possible position to prevail," a close adviser said Saturday.

Speaking remotely to a forum in Kyiv, U.S. national security adviser Jake Sullivan also said Biden will meet Ukrainian President Volodymyr Zelenskyy in late September at the U.N. General Assembly in New York to discuss aid to Ukraine.

"President Zelenskyy has said that ultimately this war has to end through negotiations, and we need them to be strong in those negotiations," Sullivan said, adding Ukraine would decide when to enter talks with Russia.