US President Donald Trump said on Thursday he had received a "beautiful letter" from Chinese President Xi Jinping as negotiations on a trade deal between the two countries continue in Washington.

"He just wrote me a beautiful letter. I've just received it and I'll probably speak to him by phone," Trump said. Later he quoted Xi as saying in the letter: "Let's work together let's see if we can get something done."

Trump said he believed it was possible to reach a deal this week, as negotiators prepare to meet again on Thursday in Washington to end a trade dispute between the world's two largest economies.

The U.S. Trade Representative's office announced that tariffs on $200 billion worth of Chinese goods would increase to 25 percent from 10 percent at 12:01 a.m. (0401) GMT on Friday, right in the middle of two days of meetings with a Chinese delegation.

Trump has accused China of reneging on commitments it had made so far in the talks.

"We're getting very close to a deal then they started to renegotiate the deal. We can't have that. We can't have that," he said.