* U.S. corn harvest progress keeps focus on ample global supply

* Wheat slips for second day as Black Sea competition weighs

* Soy at 1-month low, pressured by U.S. harvest, crush demand

PARIS/SINGAPORE, Sept 19 (Reuters) - Chicago corn slipped on Tuesday to remain at its lowest levels since December 2020 as an advancing U.S. harvest kept attention on ample international supply following a record Brazilian crop.

Wheat futures extended losses from Monday as competitive Russian prices and moves to load grain from war-torn Ukraine underscored ongoing competition from Black Sea supplies.

Soybeans slipped to a one-month low, pressured by the early stages of the U.S. harvest as well as monthly crushing data that disappointed traders.

The most-active corn contract on the Chicago Board of Trade (CBOT) was down 0.7% at $4.68-1/4 a bushel by 1135 GMT, after touching a near three-year low for a second straight session.

CBOT wheat was down 1.2% to $5.84-1/4 a bushel after an earlier one-week low. Soybeans gave up 0.4% to $13.11-1/4 a bushel after reaching their lowest since Aug. 16.

"Corn and soybeans are slipping back as harvest pressure begins in the United States," consultancy Agritel said.

U.S. government data released after Monday's market close showed that the U.S. corn harvest was 9% complete by Sunday and the soybean harvest 5% complete, both ahead of five-year averages.

The onset of harvesting has shifted attention away from parched summer weather that has contributed to crop ratings for corn and soybeans being estimated by the USDA at their lowest in a decade.

The soybean market has also been dented by a slower-than-expected August soy crush as reported on Friday by the National Oilseed Processors Association (NOPA).

Russian wheat export prices continued to decline last week, bucking a global rebound.

In Ukraine, a cargo vessel carrying grain has left the Ukrainian Black Sea port of Chornomorsk, a top government official said on Tuesday, in a test of Kyiv's ability to unblock its seaports for grain exports.

The vessel was one of two that entered Chornomorsk last week.

"Funds view the lack of action by the Russians as a sell signal," analysts at Stone said in a note.

On Monday, commodity funds were net sellers of Chicago soybean, wheat, corn, soyoil and soymeal futures, traders said.

(Reporting by Gus Trompiz in Paris, Naveen Thukral in Singapore and Peter Hobson in Canberra; editing by Subhranshu Sahu and Jason Neely)