PARIS/MANILA - Chicago corn, wheat and soybeans on Friday slid further from two-month highs hit earlier this week as South American and Black Sea supplies pressure U.S. exports, while economic uncertainty clouds demand outlook.

The most-traded wheat contract on the Chicago Board of Trade (CBOT) was down 1.0% at $8.36-3/4 a bushel by 1223 GMT. CBOT corn edged down 0.6% to $6.73-3/4 a bushel while soybeans eased 0.3% to $14.47-1/4 a bushel.

 Investors are focused on a U.S. Federal Reserve policy decision next week where the central bank is expected to deliver a hefty interest rate hike even as it risks tipping the economy into a recession.

Economic worries and an upturn in Argentine exports due to a preferential exchange-rate measure have dampened U.S. soybean prices after they rallied at the start of the week following a lower-than-expected official forecast of the U.S. soy crop. Traders now await the U.S. autumn harvest for soybeans and corn.

"On the one hand, the expected warm and dry weather is beneficial to crops, which will help American farmers to harvest early autumn," analysts at Zhongzhou Futures in China said. "On the other hand, competition from South American supplies continues to put pressure on U.S. soybean exports."

Grain markets were also monitoring comments from Russian President Vladimir Putin regarding a maritime corridor from Ukraine, which he has criticised for not helping poor countries.

The United Nations is pushing for Russian fertiliser to be included in the arrangement and Putin said on Friday Moscow was ready to provide more than 300,000 tonnes of Russian fertilisers for free to the developing world if Europe agreed to further relax sanctions on Russian exports.

Putin is attending a central Asian summit in Uzbekistan where is due to meet Turkish counterpart Tayyip Erdogan, who helped broker the Ukrainian corridor agreement.

Wheat prices have been curbed by increasing flows through the Black Sea corridor and cheaper prices for a record Russian crop, with U.S. export data on Thursday underscoring overseas competition.

"The latest weekly U.S. wheat export data published by the U.S. Department of Agriculture proved disappointing," Commerzbank analysts said.

A tentative agreement to avoid a shutdown of U.S. railroads averted disruption to exports, although traders said it also removed a short-term prospect of merchants switching to wheat for domestic livestock feed. 

(Reporting by Gus Trompiz in Paris and Enrico Dela Cruz in Manila; Editing by Neha Arora, Uttaresh.V and Vinay Dwivedi