Russia attacked Ukraine on Wednesday with cruise and ballistic missiles, as well as drones, Ukraine said. The strikes wounded at least six people in the northeastern city of Kharkiv and killed one in the region of Dnipropetrovsk, the governors there said.

Nearly three years into the war, Washington has committed $175 billion in aid for Ukraine, but it is uncertain if the aid will continue at that pace under Republican President-elect Donald Trump, who replaces Biden on Jan. 20. Trump has said he wants to bring the war to a swift end.

"The purpose of this outrageous attack was to cut off the Ukrainian people's access to heat and electricity during winter and to jeopardize the safety of its grid," Biden, a Democrat, said in a statement.

Keith Kellogg, Trump's pick for special envoy for Ukraine and Russia, also criticized Wednesday's attack.

"Christmas should be a time of peace, yet Ukraine was brutally attacked on Christmas Day," Kellogg said. "The U.S. is more resolved than ever to bring peace to the region."

During the presidential election campaign, Trump questioned the level of U.S. involvement in the conflict, suggesting European allies should bear more of the financial burden. Some of his fellow Republicans - who will control both the House of Representatives and Senate starting next month - have also cooled on sending more aid to Kyiv.

This stance - despite previous strong support in the U.S. Congress for sustained or expanded support for Ukraine - has raised concerns among Ukraine's supporters about the future of U.S. assistance under Trump.