Chinese President Xi Jinping chats with President Donald Trump during a welcome ceremony in Beijing on Nov. 9, 2017.

U.S. President Donald Trump's demand that Beijing commit to big purchases of American farm products has become a major sticking point in talks to end the Sino-U.S. trade war, according to several people briefed on the negotiations.

Trump has said publicly that China could buy as much as $50 billion of U.S. farm products, more than double the annual amount it did the year before the trade war started.

U.S. officials continue to push for that in talks, while Beijing is balking at committing to a large figure and a specific time frame. Chinese buyers would like the discretion to buy based on market conditions.

"China does not want to buy a lot of products that people here don't need or to buy something at a time when it is not in demand," an official from a Chinese state-owned company explained.

If U.S. agricultural products "enter China in a concentrated way, it might be hard for the domestic market to digest," the Chinese official added.

Oversupply of agricultural products in China would hit local prices really hard, he said, "and break the supply-demand balance."

Moreover, a massive outbreak of African swine fever has decimated the pig herd in China, battering demand for soybeans, a key feed ingredient and the biggest agricultural import from the United States.