CHICAGO, Aug 3 (Reuters) - U.S. wheat futures dropped 1.7% on Monday as rising expectations for harvests in Russia and Australia cast a shadow over already weak demand prospects for U.S. supplies, traders said.

"We got everybody in Russia talking a bigger crop estimate," said Dan Basse, president of AgResource Co in Chicago. "That has got the world's wheat market under pressure. They are the top exporter and they are going to have an abundance this year."

Russian agriculture consultancy IKAR on Monday said it had raised its forecast for Russia's 2020 wheat crop to 79.5 million tonnes from 78 million tonnes.

Soybean futures firmed to a one-week high, breaking through key technical resistance points on support from a fresh export sale that added to recent string of deals.

Corn futures edged higher, bouncing off a one-month low hit last week on bargain buying and some support from gains in the soybean market. But the decline in wheat stymied any rally attempts in corn.

At 9:52 a.m. CDT (1452 GMT), Chicago Board of Trade September soft red winter wheat futures were down 9 cents at $5.22-1/4 a bushel. "Increasing Russian production remains the focus to start the week," said Matt Ammermann, commodity risk manager at StoneX. "Australian weather is also looking more positive, raising the prospects of more wheat exports from Australia after its virtual three-year absence from export markets."

CBOT November soybean futures were 4 cents higher at $8.96-1/2 a bushel.

Private exporters reported the sale of 260,000 tonnes for delivery to unknown destinations, the U.S. Agriculture Department said on Monday.

Traders said that China, which has stepped up its buying of U.S. agricultural products in the past month, was the likely buyer.

CBOT December corn was up 1/4 cent at $3.27-1/4 a bushel. (Additional reporting by Michael Hogan in Haburg and Colin Packham in Sydney Editing by David Goodman and Steve Orlofsky)