National security advisor Jake Sullivan has served under President Joe Biden since 2021.Patrick Semansky/APBiden wants to help Ukraine before he leaves office.Jake Sullivan told ABC News that Biden aims to help Ukraine "strengthen' its position.President-elect Trump has been skeptical of the level of US aid sent to Ukraine.National security advisor Jake Sullivan on Sunday said President Joe Biden was doing what he could to "strengthen" Ukraine's position as the commander-in-chief prepares to leave office in just under two months.Sullivan made the remarks on ABC's "This Week," telling chief Washington correspondent Jonathan Karl that Biden aims to continue advancing his foreign policy objectives — which includes cease-fire discussions between Israel and Hamas — even as President-elect Donald Trump is set to be sworn back into office on January 20."We are going to do everything in our power for these 50 days to get Ukraine all the tools we possibly can to strengthen their position on the battlefield so that they'll be stronger at the negotiating table," Sullivan said.