* Grains hit by broad commodities selling, crude oil down 5.6%

* Weekly corn export sales top estimates, China buys more

* Soybeans slip as rain eases Argentina crop fears (Rewrites throughout, adds quote, updates prices, changes byline, changes dateline from PARIS/SINGAPORE)

CHICAGO, March 18 (Reuters) - U.S. corn and soybean futures fell on Thursday in a further pullback from recent highs as improving crop weather in South America and expectations for large U.S. planting increases this spring weighed on prices.

A firm U.S. dollar and tumbling energy prices, with crude oil down more than 5%, stoked further selling in broad-based declines across commodities markets.

The bearish factors overshadowed stronger-than-expected weekly corn export sales and confirmation from the U.S. Department of Agriculture (USDA) of large corn sales to China for a third straight day.

"The improvement in rains, especially in Argentina, is weighing on the trade so prices are backing off of multi-year highs," said Terry Reilly, senior commodities analyst with Futures International.

"The only bright side I see is the weekly export sales above expectations in corn. We already know what might be pencilled in for next week's report so it's buy the rumour, sell the fact today," he said.

The USDA has confirmed U.S. corn sales to China totalling nearly 3.1 million tonnes over the past three days, along with weekly sales of 1.23 million tonnes in the week ended March 11, topping trade expectations.

Chicago Board of Trade May corn fell 9 cents to $5.49 a bushel by 12:26 p.m. CDT (1726 GMT), while May soybeans dropped 17 cents to $14.00-3/4. CBOT May wheat was down 8-1/2 cents at $6.31-1/2 a bushel.

Grain markets have been under pressure this week as crop weather improved in South America following recent dryness in Argentina and excessive rains in key parts of Brazil.

Rain storms this week in Argentina's Pampas farm belt have slowed the deterioration of many drought-hit soybean and corn fields, crop weather specialists said.

Grain traders are monitoring news from a meeting between high-level U.S. and Chinese officials on Thursday.

(Additional reporting by Gus Trompiz in Paris and Naveen Thukral in Singapore. Editing by Mark Potter and Steve Orlofsky)