WASHINGTON — President Donald Trump and Chinese President Xi Jinping agreed to extend a U.S.-China trade truce until January, postponing a potential return to steep tariffs but leaving major economic disputes unresolved after their Sept. 25 summit in Washington. The extension keeps in place a fragile arrangement that had been set to expire Nov. 10. Without a deal, the two countries risked renewing tariffs and export restrictions that disrupted global markets last year. The delay gives negotiators more time while moving the next deadline beyond the U.S. midterm elections, when affordability and trade are key political issues. The summit did not announce major changes to U.S. controls on advanced chips or semiconductor equipment, or a resolution to China’s restrictions on critical-mineral exports. Those materials are used in products ranging from car batteries to defense equipment. China also remains behind on a previous commitment to buy at least $17 billion in U.S. farm products this year, excluding a separate soybean pledge, according to the administration’s account of the agreement. Trump said farmers could welcome new Chinese purchases but gave no details. U.S. Trade Representative Jamieson Greer said the countries had agreed to set aside some categories of goods in the event of future tariff disputes; he did not describe the arrangement as a broad trade overhaul. The White House offered no additional details on the farm commitments. The meetings came as high prices have weighed on the administration and some Republicans have criticized Trump’s trade policies. That political pressure may limit the White House’s willingness to use tariffs as leverage, even as U.S. companies seek greater certainty about trade with China. A proposed investment board intended to encourage cross-border business activity stalled amid security concerns and anticipated opposition from Republicans, according to people familiar with the discussions. Trump and Xi plan to meet again at the Asia-Pacific Economic Cooperation summit in China on Nov. 18 and 19, and at the Group of 20 leaders’ meeting in Doral, Florida, on Dec. 14 and 15. Those meetings offer further opportunities for negotiations, but the Washington summit left outstanding questions about market access, technology, minerals and agricultural trade.