* Wheat extends gains into fourth session
* Corn firms 1%, soybeans edge higher
* South American concerns underpin corn and soybeans
By Colin Packham
SYDNEY, Dec 14 (Reuters) - U.S. wheat futures edged higher on Monday to hit a near three-week high on expectations of strong demand for U.S. supplies as key producer Russia would soon move to restrict exports of the grain.
Corn rose more than 1%, drawing support from wheat, while soybeans also rose more than 0.5%.
The most active wheat futures on the Chicago Board Of Trade Wv1 were up 0.3% at $6.16-1/2 a bushel by 0356 GMT, near the session peak of $63.22 a bushel - the highest since Nov 25. Wheat gained 3% on Friday.
Analysts attributed the gains to expectations of increased demand for U.S. supplies.
"Russia's agriculture ministry had proposed a $30 tax on exports from mid-February to end-June," said Tobin Gorey, director of agricultural strategy, Commonwealth Bank of Australia.
"The effect will be to turn more demand to Australia and the Unites States."
Russian officials are considering imposing a wheat export tax for Feb. 15-June 30 as one measure to stabilise domestic prices, four sources familiar with discussions told Reuters. Russia's Sovecon agriculture consultancy downgraded its 2021 wheat crop forecast last week, citing the worst crop conditions in a decade.
The most active soybean futures Sv1 were up 0.7% at $11.68-1/4 a bushel, having firmed 0.7% on Friday.
The most active corn futures Cv1 were up 1.1% at $4.29 a bushel, having gained 0.5% in the previous session.
Both corn and soybeans remained underpinned by concerns over production in South America.
Widespread rains are expected in central and southern Brazil and northern Argentina next week, while a drier pattern envelops northern Brazil, weather models suggest.