CANBERRA, Sept 14 (Reuters) - Chicago wheat futures fell on Thursday as concerns over supply from war-torn Ukraine eased due to large exports of cheap grain from Russia, keeping prices near 33-month lows.

Soybean and corn futures rose slightly.

Wheat markets are expected to tighten in the coming months as the focus switches to harvests in southern hemisphere countries such as Argentina and Australia, where crops have been hit by dry weather.

But for the time being, northern hemisphere harvests, particularly Russia's, are pumping supply into the market, said Ole Houe at Australian agricultural brokerage IKON Commodities.

"That puts a lid on prices and prevents any near-term rallies," he said.

The most-active wheat contract on the Chicago Board of Trade (CBOT) was down 0.8% at $5.92-3/4 a bushel at 0419 GMT. On Friday, prices fell to $5,70 a bushel, the lowest since December 2020.

CBOT soybeans were up 0.1% at $13.50-1/2 a bushel and corn was 0.1% down at $4.81-3/4 a bushel.

Ukrainian grain exports plunged to 783,000 metric tons between Sept. 1 and Sept. 13 from around 1.5 million tons in the corresponding period in 2022, agriculture ministry data showed on Wednesday.

The ministry gave no explanation for the fall, but Ukraine's deputy prime minister said more than 100 port infrastructure facilities had been damaged by Russian attacks since July 18.

He said the export potential of Ukrainian ports on the Danube River has been cut by nearly 0.5 million tons of grain a month.

Ukraine is stepping up road and rail shipments, but volumes are smaller and costs are higher.

Exports from Russia have not slowed, however, with agricultural consultancy Sovecon this month raising its forecast for Russia's wheat exports to 48.6 million metric tons in the 2023/24 season.

Elsewhere, FranceAgriMer lowered its forecast of French soft wheat exports outside the European Union in 2023/24 by 100,000 metric tons to 9.5 million metric tons.

Argentina's Rosario grains exchange trimmed its forecast for the 2023/2024 wheat harvest to 15 million metric tons from 15.6 million tons due to dry conditions in some areas.

The U.S. Department of Agriculture (USDA) is due to release weekly export sales data on Thursday, with analysts polled by Reuters expecting net wheat sales of 250,000 to 600,000 metric tons in the week ended Sept. 7.

In soybean markets, the U.S. soybean crush in August likely increased from the same month last year, analysts said ahead of a monthly National Oilseed Processors Association (NOPA) report due on Friday.

Chicago soybeans remain near a three-week low reached on Tuesday, when the USDA cut its forecast for U.S. soybean output in a monthly report by less than some analysts had expected.

Commodity funds were net buyers of CBOT wheat soybean and corn futures on Wednesday, traders said.

(Reporting by Peter Hobson; Editing by Sherry Jacob-Phillips and Janane Venkatraman)