China's military is "likely training for strikes" against the United States and its allies, the Pentagon has claimed.
The suggestion is contained in a report that says China's People's Liberation Army (PLA) recently expanded its bomber operations.
The Pentagon's comments come at a time when the US and China are planning to hold trade talks to try to resolve an escalating dispute over tariffs.
"Over the last three years, the PLA has rapidly expanded its over water bomber operating areas, gaining experience in critical maritime regions and likely training for strikes against US and allied targets," the report said.
But it was unclear, the report added, what message Beijing was trying to send, "beyond a demonstration of improved capabilities".
China's air force has this year landed bombers on islands and reefs in the South China Sea as part of a training exercise in the disputed region.
It is estimated China spent more than $190bn (£149bn) on defence last year, with the Pentagon predicting a rise to $240bn (188bn) by 2028.
There have been mixed diplomatic signals between the two countries in recent months.
In May, the Pentagon withdrew an invitation to China to join a multinational naval exercise.
But in June, US Defence Secretary Jim Mattis became the first head of the Pentagon to visit China since 2014.
China's space programme is progressing rapidly, the report added.
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"The PLA continues to strengthen its military space capabilities despite its public stance against the militarisation of space," it said.
US President Donald Trump's administration has said it plans to bring in a new "space force" as the sixth branch of the military by 2020.