Washington (CNN) China is actively developing its fleet of long-range bombers and "likely" training its pilots for missions targeting the US, according to a new Pentagon report.

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

The "Annual Report on Military and Security Developments Involving the People's Republic of China" is a US government report mandated by Congress, which details Chinese military developments over the previous year.

This year's report also claims that China is pursuing a nuclear capability on its long-range bombers, saying the Chinese air force "has been re-assigned a nuclear mission."

"The deployment and integration of nuclear capable bombers would, for the first time, provide China with a nuclear 'triad' of delivery systems dispersed across land, sea, and air," the report said.