WASHINGTON (REUTERS) - The United States expects China to return an underwater drone "relatively soon" after a Chinese naval vessel seized it in the South China Sea last week, triggering US protests, two US defence officials told Reuters on Monday.

One scenario saw a US Navy destroyer receiving the drone from China at an agreed upon location in the South China Sea, one of the officials said. Both officials, who spoke on condition of anonymity, said talks with China were progressing.

The Pentagon says the unmanned, underwater vehicle (UUV) used unclassified, commercially available technology to collect scientific data about things like the salinity, temperature and clarity of the water.