Mexico has emerged as China's biggest obstacle to joining the WTO, the organisation that sets rules on international trade. It is the only WTO member that hasn't reached a bilateral trade pact with China, possibly holding up the Asian country's WTO entry. Analysts are surprised by Mexico's tough stance. "This has actually dragged on longer than I thought it would. Once China had a bilateral agreement with the United States, I thought it was a slam dunk," said Tom Byrne, a lead analyst for China and East Asia at Moody's Investors Service, based in New York.