UPDATE 7-U.S., China to keep talking on trade after little progress in Shanghai

* The two sides to reconvene in September in Washington

* China says talks were "candid, highly effective"

* White House says China restates commitment to buy farm goods

* Chinese spokeswoman says U.S. continues to "flip-flop" (Adds comments from sources on focus of talks)

By Brenda Goh, David Stanway and Andrea Shalal

SHANGHAI/WASHINGTON, July 31 (Reuters) - U.S. and Chinese negotiators ended a brief round of trade talks on Wednesday with little sign of progress and agreed to meet again in September, prolonging an uneasy truce in a year-long trade war between the world's two largest economies.

The talks were the first face-to-face meetings since U.S. President Donald Trump and Chinese President Xi Jinping met in June and agreed to get negotiators back together to try to find a way out of the dispute.

The White House and China's Commerce Ministry each described the meetings in Shanghai as constructive, but neither announced any agreements or goodwill gestures that might have cleared the path to more substantive future talks.

The two governments have levied billions of dollars of tariffs on each other's goods, disrupting global supply chains and roiling financial markets. The International Monetary Fund has warned that the trade dispute will shave 0.2% off global output.

Concern that the lack of progress could point to a protracted trade war weighed on global markets on Wednesday.

Two people familiar with the talks said they focused largely on goodwill gestures including Chinese purchases of U.S. soybeans, pork, ethanol and other commodities, and U.S. moves to relax restrictions on sales to Chinese telecommunications giant Huawei.

The month-long delay until the next meeting gives both sides time to take action on these commitments, the Washington-based sources said.

U.S. Trade Representative Robert Lighthizer and Treasury Secretary Steven Mnuchin left China with little in hand save a pledge to keep talking, following a half-day meeting and a working dinner at Shanghai's historic Fairmont Peace Hotel.

China's Commerce Ministry said "both sides ... had a candid, highly effective, constructive and deep exchange on major trade and economic issues of mutual interest." It said negotiators discussed more Chinese purchases of U.S. agricultural products, but did not say there was any agreement to buy more.

The White House said China restated its commitment to buy more U.S. farm goods and said negotiations on "an enforceable trade deal" would continue in Washington in early September, but gave no details about expected agricultural purchases.