* Argentina bars corn export sales through February

* Dry weather threatens crops in Argentina, southern Brazil

* Corn, soy eye biggest annual gains in at least a decade (Rewrites throughout with U.S. market open, adds quote, updates prices, changes byline, changes dateline from SHANGHAI)

CHICAGO, Dec 31 (Reuters) - U.S. corn futures extended gains on Thursday and hit their highest level in 6-1/2 years after major supplier Argentina said it would curb exports and as dry weather threatened harvests in South America.

Soybeans also scaled a fresh 6-1/2-year peak on South American weather worries and tightening global supplies, while wheat eased on profit-taking after hitting a six-year high.

All three commodities were poised for solid gains for the week, month, quarter and year.

After climbing for 14 straight sessions, corn is set for its largest annual gain in a decade, adding more than 23%. Soybeans are eyeing a 37% annual gain, their strongest year since 2007. Wheat's nearly 14% gain for 2020 would be the fourth straight year in the black.

Corn guided grains higher on Thursday after Argentina's agriculture ministry announced that the country would suspend sales of corn for exports until Feb. 28 to ensure ample domestic food supplies.

The move triggered more buying in a futures market already on edge due to crop-threatening South American weather amid robust demand and tightening global corn and soy supplies.

"There is continued concern about Argentine and southern Brazilian weather," said Jack Scoville, analyst with the Price Group. "There's also a lot of fund buying just based on the trends, which are extremely up."

Argentina and southern Brazil are expected to remain hot and dry for much of the next two weeks, according to meteorologists.

Chicago Board of Trade March corn jumped 6 cents to $4.80-1/2 a bushel by 10:25 a.m. CST (1625 GMT), the highest for a most-active contract since May 22, 2014.

March soybeans gained 12-1/2 cents to $13.13 a bushel, the loftiest level for a most-active contract since June 23, 2014.

CBOT March wheat was down 1-3/4 cents at $6.39 a bushel after hitting a six-year high of $6.44-1/2 overnight. (Additional reporting by Emily Chow in Shanghai; Editing by Subhranshu Sahu, Vinay Dwivedi and Jonathan Oatis)