PARIS - Rain in large parts of France overnight provided some relief but not enough for cereals suffering a prolonged drought, the head of the France's largest farm union said on Monday.

Dry weather in France, the European Union's biggest grain producer, has added to market worries about global supply due to war disruption in Ukraine, a major grain exporter. A heat wave harming crops pushed India to ban wheat exports on Saturday.

FNSEA chief Christiane Lambert said on BFM television that the bone-dry soil prevented crop from absorbing much of the rain.

"The situation is very complicated because water reserves are low, there is a 25% deficit. Therefore, the soil is very dry and the rain, when it arrives suddenly, does not manage to descend into the soil but washes away," she said.

Grass would benefit, but cereal crops remain in a difficult situation, she said.

"For wheat, for cereals, it is now that the grains must fatten and now that they need water regularly," she said.

Groundwater levels worsened rapidly since February in France due to insufficient water with some parts of the country at worrying levels, mainly in western and southeastern France, French Geological Survey BRGM said on Friday.

Crop data released on Friday showed conditions for French wheat and barley crops had declined as dry weather persisted.

Lambert noted that a fall in French wheat production would come at a time when the outlook for grains is difficult worldwide.

U.S. wheat conditions are rated among the lowest in years due to dryness.

Lambert also pointed to sowing problems in Ukraine, one of the world's largest grain exporters, and an expected fall in yields in the country.

"We are seeing shortages everywhere," she said.

(Reporting by Geert De Clercq and Sybille de La Hamaide; Editing by Edmund Blair and Lisa Shumaker)