* U.S. confirms corn sale to China, largest since Feb
* Corn also sold to "unknown," believed to be China
* U.S. soybeans also confirmed sold to China
* USDA's current China corn import forecast seen too low (Recasts, adds links to graphics, adds byline)
By Karl Plume
March 29 (Reuters) - China bought 120,000 tonnes of U.S. corn for shipment by the end of August, the largest in a month, the U.S. government said on Thursday, bringing the country's total confirmed purchases of the grain this season to nearly 4 million tonnes.
More old-crop sales to the world's No. 2 corn consumer were expected so the latest U.S. Department of Agriculture forecast for imports by China was likely too low, suggesting U.S. stocks of the grain, already seen shrinking to a 16-year low by the next harvest, could tighten further.
An additional sale of 120,000 tonnes to an "unknown" destination for shipment in the next September-to-August marketing year was also confirmed by the USDA on Thursday. That sale was widely believed to be to China as well, analysts and traders said.
Private sources on Wednesday confirmed sales of six cargoes of old-crop U.S. corn to China and there have also been unconfirmed reports of additional demand by the country its domestic prices were at a steep premium to the cost of imported grain.
China, historically self sufficient in corn, has become a large importer in recent years, buying around 1 million tonnes in each of the past two years, as demand has outpaced its domestic production.
The country was forecast by the USDA to import a 17-year-high 4 million tonnes in the 2011/12 marketing year, a figure which traders and analysts expect will be revised upward, possibly topping the record 4.287 million tonnes imported in 1994/95.
Confirmed sales to China for 2011/12 shipment now total about 3.92 million tonnes, according to USDA data. That total is likely much higher as some of the sales to "unknown" destinations are earmarked for China.
Private exporters also reported the sale of 120,000 tonnes of U.S. soybeans to China for 2011/12 delivery, USDA said. The 2012/13 marketing year for corn and soybeans opens on Sept. 1. (Additional reporting By Emily Stephenson in Washington; Editing by Marguerita Choy)