US Republican presidential candidate Donald Trump pauses while delivering a campaign speech about national security in Manchester, New Hampshire. Reuters

US President-elect Donald Trump railed against China in a series of tweets on Sunday night, accusing the Asian giant of currency manipulation and flexing its military might in the South China Sea.

"Did China ask us if it was OK to devalue their currency (making it hard for our companies to compete), heavily tax our products going into their country (the U.S. doesn't tax them) or to build a massive military complex in the middle of the South China Sea? I don't think so!"

It was not clear what prompted the tweets, which came amid a controversy over a phone conversation Trump held with the leader of Taiwan, a self-ruled island Beijing considers part of its territory.