China's production of refined copper may have been inflated by more than 15 percent this year, smelter sources said.

China is the world's top copper producer and importer and a pumped-up production figure would exaggerate its underlying demand.

Nine of China's top smelters met earlier this month to seek clarity on supply and demand fundamentals after China's National Bureau of Statistics reported record refined copper output in October and annualized 2013 production of 6.8 million tons.

Executives from three large smelters, who will meet global miners for term contract talks later this year, told Reuters the official headline figure had likely been boosted by double-counting, as well as products from smaller plants being wrongly categorized.

"Our production has been double-counted and other producers should be in the same situation," said one of the executives, who declined to be named given the sensitivity of the issue.

The three estimated China's actual 2013 refined copper output at about 5.6 million tons, 17 percent below the official figure, based on production schedules at 31 firms. They forecast 2014 output at about 6.3 million tons.

"Based on our refined output estimate, there has not been as much supply in the domestic market as the statistics indicate," one of the executives said.

"The fact is that tens of thousands of tons of bonded stocks have already been used because of a supply shortage," he added, referring to a fall in Shanghai stocks from 1 million tons in the first quarter to around 400,000 tons currently.

The executives said copper output figures have been inflated for years, but the problem is getting worse as smelters expand operations to other provinces.

In one example, Jiangxi Copper Corp, China's top producer, included output at its 100,000-tons-a-year Penghui unit in Shandong Province when reporting production to the Jiangxi government, where the firm is headquartered. However, the Penghui subsidiary also reported its own output to Shandong authorities, said two people familiar with operations at the firms.