MEXICO CITY, March 28 (Reuters) - U.S. wheat and soybean futures closed higher on Tuesday and corn closed nearly flat as traders had doubts about planting delays and adverse weather with an eye toward a key U.S. crop seedings report at the end of the week.

Traders squared positions as they looked ahead to Friday's annual U.S. Agriculture Department (USDA) planting intentions report.

Analysts polled by Reuters ahead of the report on average expect 2023 corn plantings at 90.880 million acres, wheat at 48.852 million acres and soybeans at 88.242 million acres.

Spring wheat gained on speculation over planting delays in the northern U.S. Plains due to cold weather, analysts said.

"The bearish window is ending for the grain markets," said Craig Turner of Daniels Trading. "We're going to start seeing more of a bullish trend."

Soybeans were also supported by technical trading and a bleak outlook on harvesting in drought-stricken Argentina, traders said.

The most active wheat contract on the Chicago Board of Trade (CBOT) settled up 1-3/4 cent at $6.99-3/4 per bushel.

Soybeans closed at $14.67-3/4 per bushel, up 25-1/2 cents or 1.8%, the most-active contract's strongest percentage gain in 4-1/2 months. Corn settled down 1 cent at $6.47-1/4 per bushel.

The USDA reported another flash sale to China of 136,000 tonnes of U.S. corn.

Continued uncertainties surrounded the Black Sea corridor and export volumes.

Ukraine's 2022/23 grain exports fell 17.7% to 36.9 million tonnes as of March 27, reflecting a smaller harvest and logistical difficulties, agriculture ministry data showed.

"Ukraine's only planting about two-thirds of their wheat that they normally do, and when you look at the stock, the usage for next year, wheat looks just as tight as it did the year before," Turner added.

Russia has no plans to halt wheat exports but wants exporters to ensure high prices for farmers, sources told Reuters on Friday. (Reporting by Cassandra Garrison in Mexico City, Mei Mei Chu in Kuaka Lumpur and Sybille de La Hamaide in Paris; Editing by Marguerita Choy)