President Biden will begin what is likely to be his last summit with global leaders as commander in chief on Monday, pushing for even stronger support of Ukraine despite the looming uncertainty of how president-elect Donald J. Trump might undo his efforts.

Just before the Group of 20 summit began in Rio de Janeiro, Mr. Biden authorized the first use of U.S.-supplied long-range missiles by Ukraine for strikes inside Russia, U.S. officials said.

The decision, a major change in U.S. policy, was made in part to help shore up Ukraine’s defenses after Russia recruited North Korean troops to assist its fighters. It also came on a day when Russia bombarded Ukraine’s power grid in one of the largest attacks of the war.

For two years, Mr. Biden sent billions of dollars in military aid to Ukraine to help repel Russia, but repeatedly hesitated when it came to offensive weapons, worried about provoking President Vladimir V. Putin of Russia into a wider war.

Mr. Putin, like Mr. Trump, will not be at the summit. But unlike Mr. Biden, both men are sure to be at the center of conversations about a future they will both dominate.

President Emmanuel Macron of France, who is in Rio for the meeting, told reporters that the Russian bombardment of Ukraine on Sunday shows that Mr. Putin “does not want peace and is not ready to negotiate.”