The United States on Friday urged Japan and South Korea to speak out against China's treatment of minority Muslims and its actions toward Hong Kong and Taiwan, despite their trading relationships with Beijing.

The unusually blunt comments directed at close U.S. allies came from Marc Knapper, deputy assistant secretary for Korea and Japan, as Tokyo and Seoul were set to join 13 other Asia Pacific economies this weekend in signing a China-backed Regional Comprehensive Economic Partnership (RCEP), which could become the world's largest free trade agreement.

Knapper also stressed the importance of South Korea, Japan, Taiwan and Southeast Asian countries working with the United States and each other to ensure their citizens' data was protected from China.