The decision was taken "to put America's interests first" and followed a Department of Defense review of US "military support and assistance to other countries", White House spokesperson Anna Kelly said on Tuesday.
The US has sent tens of billions of dollars in military aid to Ukraine since Russia launched its full-scale invasion in February 2022, leading some in the Trump administration to voice concerns that US stockpiles are too low.
The Ukrainian government has not commented on the announcement. US officials did not immediately say which shipments were being halted.
Air defence missiles and precision munitions are understood to be among the weapons affected, according to the Reuters news agency.
The Department of Defense "continues to provide the President with robust options to continue military aid to Ukraine," said Elbridge Colby, the US Undersecretary of Defense for Policy.
At the same time, Colby added, "the Department is rigorously examining and adapting its approach to achieving this objective while also preserving U.S. forces' readiness for Administration defense priorities."