President Donald Trump boasted Sunday without providing evidence that China had "already begun" making large purchases of U.S. agricultural products as part of last week's tariff deal with Beijing.

Trump announced the partial agreement on Friday, calling it a "substantial phase-one deal" that would take three to five weeks to write. He offered no details on Sunday to support his assertion that China had already begun making purchases weeks before terms of the deal are codified.

The United States had planned to raise tariffs on $250 billion of Chinese imports from 25 percent to 30 percent on Tuesday, but Treasury Secretary Steven Mnuchin said Friday that the increase wouldn't take place.

The United States is China's biggest export market, but U.S. purchases of Chinese goods have plummeted since the punitive U.S. tariffs on Chinese goods went into effect over the summer, targeting China's growing strength in the technology sector.

China reported Monday that exports to the United States fell by 17.8 percent in September — after a decline of 16 percent in August, when the U.S. tariffs took effect, The Associated Press reported.

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"My deal with China is that they will IMMEDIATELY start buying very large quantities of our Agricultural Product, not wait until the deal is signed over the next 3 or 4 weeks," Trump tweeted on Sunday. He said on Friday that the agricultural purchases would eventually total $40 billion to $50 billion.

My deal with China is that they will IMMEDIATELY start buying very large quantities of our Agricultural Product, not wait until the deal is signed over the next 3 or 4 weeks. THEY HAVE ALREADY STARTED! Likewise financial services and other deal aspects, start preparing.... — Donald J. Trump (@realDonaldTrump) October 13, 2019

Mnuchin said no decision had been made on whether to kill or delay another round of tariffs scheduled to take effect on Dec. 15. Trump has said talks on a "phase two" deal will start almost immediately.

Asian markets appeared to approve of the deal on Monday. The main Shanghai composite index was up by about 1½ percent in early trading, while South Korea's KOSPI stock market was up by about 1¼ percent and Australia's ASX 200 was up by almost three-quarters of a point. CNBC attributed the rises to investors' confidence in the trade agreement.

Trump's promotion of the tariff arrangement comes only a week and a half after he called on the Chinese government to investigate former Vice President Joe Biden and his son, Hunter, over Hunter Biden's involvement with a private equity fund that has raised money in China.

An attorney for Hunter Biden said Sunday that his client would step down from the fund and that he would avoid future work that might pose conflicts of interest should his father win the White House.