The purchases from Brazil, rather than the United States, show that Chinese buying has been driven more by price than policy since last week's preliminary trade agreement that U.S. President Donald Trump hopes will be signed next month.

Anticipation that Chinese buyers would return to the U.S. market to make big purchases in the wake of the deal drove benchmark U.S. soy prices last Friday to their highest levels since the start of the trade war more than 15 months ago.

The rally in U.S. prices last week, however, made Brazilian soy more of a bargain for Chinese buyers. Brazilian soy is even more appealing to commercial importers because a 25% duty on U.S. shipments remains in effect and Beijing has not awarded the non-state-owned firms any new tariff waivers.

Since Monday, two traders said, China has booked at least eight boatloads, or 480,000 tonnes worth $173 million, of Brazilian soybeans. While Brazil is China's largest soybean supplier, large purchases from South America are unusual at this time of year with the U.S. harvest coming in.

Benchmark U.S. prices posted their first weekly decline in three weeks as Chinese buying failed to materialise in the U.S. market.

Trump said on Twitter on Sunday that China has already begun making U.S. agricultural purchases. But three U.S. soybean exporters said there have been no U.S. sales to China since last week's talks in Washington, and none have been confirmed by the U.S. Department of Agriculture.

"I've not had any inquiries at all for U.S. (shipments)," said one of the U.S. soybean exporters. "There were a few November boats bought from Brazil and several new-crop South American boats for March forward but nothing here."

Another U.S. exporter said a drop in Brazilian soybean prices sparked fresh demand from commercial soy importers that have been unable to profitably import American soybeans for more than a year unless given tariff waivers.

State-owned firms COFCO and Sinograin, which are exempt from the 25% retaliatory duties on U.S. imports, have "little appetite" to buy unless U.S. prices drop further, the second U.S. exporter said.

White House economic adviser Larry Kudlow acknowledged on Thursday that China's "serious commitment" to buy up to $50 billion of agricultural products would depend in part on market conditions.

Before the trade war, China imported most of its U.S. soy between October and January, turning to South America around February.

Prices for U.S. soybeans loaded at Gulf Coast and Pacific Northwest terminals in November and December and shipped to China are now near par with Brazilian soy prices. But when prices for soybeans from the two top suppliers are similar, Chinese importers tend to favour Brazilian beans because of their higher average protein content.

Chinese importer Hopefull Grain & Oil bought 10 cargoes of Brazilian soy last week ahead of U.S.-China talks and at least three cargoes this week, two of the trade sources said.

Wilmar was also a buyer, with about five to six cargoes purchased from Brazil this week, according to a U.S. exporter and two traders, one of whom was based in Beijing while the other worked for a Chinese trading house.

Both Hopefull and Wilmar declined to comment.

The companies are believed to have used up their waivers for tariff-free U.S. purchases in recent waves of buying, a U.S. exporter and a Chinese importer said.

Chinese importers bought about 850,500 tonnes in U.S. soy in the week ended Oct. 10, just ahead of the phase one deal announcement, according to weekly USDA data released on Friday.

Total Chinese purchases have topped 4.5 million tonnes since early September, when Beijing began awarding tariff waivers to some commercial importers, USDA data showed.

(Reporting by Karl Plume in Chicago, Hallie Gu and Dominique Patton in Beijing, Naveen Thukral in Singapore, and Roberto Samora in Sao Paulo; Editing by Simon Cameron-Moore and Steve Orlofsky)

By Karl Plume and Hallie Gu