Ukrainian President Volodymyr Zelenskyy says military aid from the US and other countries has helped to strengthen his army, enabling it to defeat Russian forces and show that its larger neighbour is not invincible.
Mr Zelenskyy is also pressing allies to impose tougher and wider sanctions on Russia, saying such painful measures will help to undercut public support for Russian President Vladimir Putin's war in Ukraine.
"How Ukraine can be helped?" he asked. "There are ways many countries worldwide could help Ukraine, through artillery pieces that we need, ammunition, and definitely the US could have political influence in these countries to convince them.
Mr Zelenskyy was speaking online at a Clinton Foundation event hosted by former president Bill Clinton at the UN General Assembly in New York.
The Ukrainian leader was last week granted approval by the General Assembly to address world leaders online this week, making an exception to its requirement that all leaders speak in person.
Russia is preparing to hold referendums this week in regions over which it has control in eastern Ukraine, and some areas where it does not.
The referendums will ostensibly decide whether those regions want to become part of Russia.
"In this war you cannot remain on the sidelines because we have a real occupation, a real war of aggression Russia has waged against us," Mr Zelenskyy said.
"And you definitely have to take sides. You cannot vacillate between good and evil, or the light or dark."
AP contributed to this report