US President-elect Donald Trump has posted a series of tweets criticising China for its monetary policy and its operations in the South China Sea.
"Did China ask us if it was OK to devalue their currency" and "build a massive military complex?" he asked. "I don't think so!"
Last week Mr Trump risked a diplomatic rift with China by speaking directly with Taiwan's president.
The highly unusual move saw China lodge a complaint with the US.
The US has previously criticised China's yuan devaluation, saying it unfairly favours Chinese exporters.
It has also told Beijing to stop reclaiming land around islands and reefs which are claimed by multiple countries in the South China Sea, and has sent US Navy ships to the area. Both sides have accused each other of "militarising" the region.
China has said its land reclamation efforts are for civilian purposes.