* Rapidly progressing U.S. soybean, corn planting weighs on prices

* Uncertainty over Black Sea export deal as expiry date approaches

SINGAPORE, May 9 (Reuters) - Chicago soybean and corn futures lost ground on Tuesday with rapidly progressing U.S. planting boosting expectations of bumper production and weighing on prices.

Wheat slid for a second session, although losses were limited by the poor condition of the U.S. winter crop and an uncertainty over Ukraine's safe shipping agreement for grain exports that is set to expire on May 18.

"U.S. Midwest weather in beneficial for soybean and corn planting," said one Singapore-based trader. "This is bearish for prices."

The most-active soybean contract on the Chicago Board of Trade (CBOT) was down 0.4% to 14.27-3/4 a bushel, as of 0245 GMT, and corn dropped 1% to $5.90-1/2 a bushel.

Wheat lost 1.2% to $6.46 a bushel.

Rapidly progressing corn and soybean planting weighed on prices.

The U.S. Department of Agriculture (USDA) reported corn and soybean planting progress that topped market expectations, with the pace of soybean seeding the second fastest on record.

Farmers had planted 49% of their corn and 35% of their intended soybean acreage, the agency said.

For wheat, U.S. winter crop conditions improved by less than expected in the past week despite rains in key growing areas.

Good-to-excellent ratings for the U.S. winter wheat crop rose 1 percentage point to 29% in the week ended May 7, the USDA said in its weekly Crop Progress and Conditions Report. That matched 2022 as the lowest good-to-excellent rating for this time of year since 1996.

Russia has effectively stopped the Black Sea grain deal by refusing to register incoming vessels, Ukraine's reconstruction ministry said on Monday.

The deal expires on May 18. Moscow has threatened to quit the agreement, while Turkey and the United Nations (UN) are working to extend it.

The UN said no ships were inspected on Sunday or Monday under the deal.

Forward sales of Brazil's second corn crop hit 24.3% of the expected production of 92.2 million tonnes, according to a survey by agribusiness consultancy Safras & Mercado that reflects slow farmer selling for this time in the season.

Commodity funds were net sellers of CBOT grain and soybean futures contracts on Monday, traders said. (Reporting by Naveen Thukral; Editing by Sherry Jacob-Phillips and Sonia Cheema)