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 senior adviser said Saturday.

Speaking remotely to a forum in Kyiv, Ukraine, Jake Sullivan, the U.S. national security adviser, 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.

Biden will be replaced next January either by Vice President Kamala Harris, who has indicated she will continue his policies of backing Ukraine, or by former President Donald Trump, who would not say at a debate earlier this week whether he wanted Kyiv to win the war.

The announcement of the upcoming Biden-Zelenskyy meeting came after Moscow and Kyiv earlier Saturday swapped 103 prisoners of war each in a UAE-brokered deal, and as Russian forces continue to gain ground in their grinding offensive in east Ukraine.

Sullivan, in his comments by video link to the forum in Kyiv, said "difficult and complicated" logistics — rather than unwillingness — was delaying aid to Ukraine.

"It's not a matter of political will," Sullivan said. "But given what Ukraine is up against, we've got to do more, and we've got to do better."