* Chicago wheat erases big gains
* Brazil rain forecasts in focus for soybeans, corn
* Corn down 0.7%, soybeans fall 0.5%
(New throughout, updates prices, market activity, US trading comments; changes dateline, previous PARIS/SINGAPORE)
CHICAGO, Dec 27 (Reuters) - Chicago wheat declined on Wednesday, erasing most of the gains made the day before when worries that escalating wartime tensions would impact shipping in the Black Sea exporting region sent grains markets higher.
Soybeans and corn also moved lower on Wednesday as traders assessed a rainy forecast in South America, after drought has exacerbated concerns over crop yield in top soy and corn exporter Brazil.
The most-active wheat contract on the Chicago Board of Trade (CBOT) was down 2.2% at $6.22-1/2 a bushel by 10:30 a.m. (1630 GMT). Without signs of strong demand, the contract wiped out most of the gains of Tuesday, when it closed more than 3% higher after Ukraine struck a large Russian landing warship.
"The demand isn't good enough to move the market itself," said Mark Schultz, chief analyst at Northstar Commodities. "It's not a demand-driven market, on the grains or oil seeds, at this juncture, it's just not."
CBOT soybeans fell 0.57% to $13.11-1/2 a bushel and corn gave up 0.78% to $4.76-1/2 a bushel.
Weather charts showed uneven showers in the week ahead in dry parts of central and northern Brazil, before widespread heavy rain expected in early January.
For corn, precipitation in the coming weeks will also influence yields in the upcoming first-crop harvest and affect planting prospects for the larger second crop in Brazil.
"Otherwise there haven't been any significant sales being announced to give you some type of encouragement or something else besides just watching the weather day in and day out," Schultz said, referring to a lack of announcements by the U.S. Department of Agriculture of export sales to China and elsewhere this week. (Additional reporting by Gus Trompiz in Paris and Naveen Thukral in Singapore; editing by Barbara Lewis)
