"The US military could suffer unacceptably high casualties and loss of major capital assets in its next conflict. It might struggle to win, or perhaps lose, a war against China or Russia," said the report prepared by the National Defence Strategy Commission, a bipartisan panel of experts created by Congress.
The commission warned that if the United States did not act swiftly to remedy the situation, the consequences would be "grave and lasting."
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The report warned of the increasing threat from not only major powers such as Russia and China, but also other countries such as Iran and North Korea.
"Those countries have developed more advanced weapons and creatively employed asymmetric tactics," it said.
In addition, the US faces an intensifying threat from transnational terrorist organizations, particularly radical jihadist groups.
"Around the world, the proliferation of advanced technology is allowing more actors to contest US military power in more threatening ways,” the report said.
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The body is tasked with making its recommendations to the president, secretary of defence and Congress. The report was submitted to Congress on Tuesday and made available to the public on Wednesday.
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