"How does he know what's going on? He was there for two weeks and he's an expert in USAID? Give me a break," said former USAID Administrator Andrew Natsios.

China's tariffs - due to begin next Monday - would also apply to US oil and farm machinery, but more talks are due this week.

North Korea is criticizing Secretary of State Marco Rubio’s description of the country as a "rogue state," calling it "nonsense" and vowing to push back.