China's military has expanded overwater bomber operations farther from the country's shores, likely training for strikes against the United States and its allies, including Japan, the U.S. Defense Department said Thursday.

In an annual report to Congress on military and security developments involving China, the Pentagon referred to an August 2017 flight by six People's Liberation Army Air Force H-6K bombers through the Miyako Strait, south of Okinawa's main island, which for the first time turned north to fly east of Okinawa and as far north as the Kii Peninsula in western Japan.

"These flights demonstrated a maturing capability for H-6K bombers to conduct off-axis strikes against U.S. and allies facilities," the report said.