Soldiers of China's People's Liberation Army (PLA) Navy patrol at Woody Island, in the Paracel Archipelago, which is known in China as the Xisha Islands, January 29, 2016. The words on the rock read, "Xisha Old Dragon". Old Dragon is the local name of a pile of rocks near Woody Island.

China's military has expanded its bomber operations in recent years while "likely training for strikes" against the United States and its allies, a Pentagon report released on Thursday said.

The assessment, which comes at a time of heightened U.S.-China tensions over trade, was contained in an annual report that highlighted China's efforts to increase its global influence, with defense spending that the Pentagon estimates exceeded $190 billion in 2017.

"Over the last three years, the PLA has rapidly expanded its overwater bomber operating areas, gaining experience in critical maritime regions and likely training for strikes against U.S. and allied targets," the report said, using an acronym for China's People's Liberation Army.

The report comes as China and the United States plan to hold trade talks, offering hope they might resolve an escalating tariff conflict that threatens to degenerate into an all-out trade war.

The report said that while the PLA had continued to extend operations, it was not clear what message Beijing was seeking to send by carrying out the flights "beyond a demonstration of improved capabilities."

The Chinese embassy in Washington could not immediately be reached for comment.

This year China's air force landed bombers on islands and reefs in the South China Sea as part of a training exercise in the disputed region.