Days before a U.S.-China trade deal is due to be signed, large Chinese purchases of Brazilian soybeans and a pair of unexpected policy moves by Beijing have dimmed U.S. hopes that China would double its imports of American farm products this year.

U.S. President Donald Trump has touted the prospect of China buying $40 billion in U.S. farm products in 2020 as a pillar of the "Phase 1" agreement aimed at defusing the bitter trade war that erupted between the world's two largest economies in 2018.

The conflict, marked by tit-for-tat tariffs, has disrupted the flow of billions of dollars in goods and threatened to slow global growth, rattling financial markets. It has also sent a chill through the U.S. farm community, a key political constituency for Trump as he seeks re-election in November.