SINGAPORE/PARIS - Chicago corn futures rose for a third straight session on Tuesday, supported by a slower than expected U.S. harvest, while continuing concern over global supplies pushed wheat back into positive territory after a fall the previous day.

The fall of the U.S. dollar was also supportive for U.S. grain futures. The most active corn contract on the Chicago Board of Trade (CBOT) rose 0.4% to $6.83-1/4 a bushel by 1020 GMT. Wheat was up 0.7% at $9.18 a bushel and soybeans gained 0.4% to $13.79-1/4 a bushel.

The U.S. corn harvest was 20% complete as of Sunday, the U.S. Department of Agriculture (USDA) said in a weekly crop progress report on Monday, trailing an average estimate of 22% in a Reuters analyst poll and the five-year average, also 22%.

The soybean harvest was farther ahead at 22% complete, surpassing the average analyst estimate of 20% but behind the five-year average of 25%.

Heightened tensions between Russia and Ukraine, the world's leading grain exporters, triggered worries over supplies. Ukraine's winter wheat sowing for the 2023 harvest has covered only a third of the area completed at the same time last year, agriculture ministry data showed on Tuesday.

Ukrainian forces have broken through Russia's defences in the south of the country while expanding their rapid offensive in the east, seizing back more territory in areas annexed by Moscow and threatening supply lines for Russian troops.

The USDA, meanwhile, said the 2022 U.S. wheat harvest was smaller than previously forecast and cut its crop assessment below analyst expectations to 1.650 billion bushels.

Commodity funds were net buyers of CBOT corn, soybean, soyoil and soymeal futures contracts on Monday and net sellers of wheat futures, traders said. The dollar slid on Tuesday as U.S. treasury yields paused in their relentless climb higher, helping the euro and sterling move further off multi-year lows. 

(Reporting by Naveen Thukral and Sybille de La Hamaide, Editing by Subhranshu Sahu and David Goodman)