The World Trade Organization on Friday partly upheld a Chinese appeal in a ruling on U.S. anti-dumping measures on some steel pipes, tyres and laminated woven sacks that left Washington smarting.
Washington said it was "deeply troubled" by the outcome while China claimed a "major victory."
A WTO dispute settlement panel had rejected parts of the original Chinese complaint against the United States in a mixed and highly complex ruling delivered last October.
But the Geneva-based trade watchdog's appeals body on Friday reversed at least two key points of those findings following Beijing's appeal in the latest episode in a series of bruising trade battles between the economic giants.
It called on the United States "to bring its measures found... to be inconsistent" with the WTO's agreed rules "into conformity with its obligations under that agreement," according to the 232-page report by the appellate body.
"I am deeply troubled by this report," U.S. Trade Representative Ron Kirk said in an e-mailed statement.
"It appears to be a clear case of overreaching by the appellate body," he added. "We are reviewing the findings closely in order to understand fully their implications."
Last October, the arbitrators had upheld China's complaint against the U.S. Department of Commerce on five elements of the case, but had rejected China's contentions on several more points.
China welcomed the outcome of its appeal, which covers the lucrative trade in materials used in a host of areas from plumbing to fencing and cheap car tyres.
"The report found that the measures taken by the United States against China flouted world trade rules," a statement published online by China's commerce ministry said, calling it "a major victory."
The 153 WTO member states have a right to levy extra duty specifically targeting excessively cheap imports after their own probe, although the anti-dumping measures may subsequently be challenged in the WTO by the exporting nation.
When Beijing formally lodged the complaint in September 2008, it targeted "unfair practices" that Washington applied during the U.S. investigation that produced the extra duties on Chinese goods in 2007.
The WTO had admitted last October that its independent legal and trade experts had delayed their verdict by about a year as they grappled with the "substantive complexity" of the case.
Fast growing China's gradually opening economy and its supported export industries have been targeted by a series of complaints by rival trade powers, the European Union and the United States.
However, China has also made increasing use of the WTO's settlement system, which is aimed at avoiding all-out trade wars, since it joined the global body about a decade ago.
Beijing has so far lodged eight complaints, six against the United States and two against the European Union.
Copyright Agence France-Presse, 2011