BUENOS AIRES, April 15 (Reuters) - Prolonged rainfall over Argentina's farming heartland has fueled fears of delays to the ongoing soybean harvest that could cause production losses, the Rosario grains exchange said on Monday.

Since Friday, at least 70 millimeters (2.76 inches) of rain fell across most of the agricultural region, with peaks of 140 millimeters in some parts.

"There is beginning to be a lot of concern about the soybean harvest because there are many areas that have not finished drying," said Cristian Russo, the exchange's head of agricultural estimates.

Argentina is a top global soybean producer.

The rainfall adds to March's higher-than-normal 200-400 millimeters of precipitation - a stark contrast from the historic drought that dragged down last season's soy crop.

The heavy rains make it difficult for harvesting machines to enter fields, with the risk of losses from diseases or the ripening of soybeans in their pods.

"Everything will depend on whether this phenomenon passes and whether we have rapid drying conditions," said Russo.

Last week, Russo reduced his estimate for the 2023/24 soybean harvest, which began in March, to 51 million metric tons due to a heat wave in January and February.

Argentina's National Meteorological Service forecasts a respite from the rains in the Pampas region starting on Tuesday, followed by five days of dry and sunny to partly cloudy conditions.

Farmers harvested nearly 11% of the soybean-planted land up until the rains started last Thursday, according to the Buenos Aires grain exchange.

Adverse conditions for soybeans could boost Argentina's 2024/25 wheat season, which farmers will begin sowing next month.

Russo said the rains could also assist wheat planting, as well as for next season's corn and soybean crops, with planting for both set to begin in September.

The rains could offer a buffer as Argentina awaits the weather phenomenon known as La Nina, which begins in June and is expected to trigger drier conditions. (Reporting by Maximillian Heath; Editing by Leslie Adler)