PARIS, Aug 1 (Reuters) - Ukrainian farmers are holding back on orders for rapeseed to sow for next year's harvest as Russia's invasion continues to disrupt agricultural activity, French seed group Vilmorin said on Monday.

Ukraine's president said on Sunday that this year's harvest could be half its usual size, and some farmers and observers fear next year's production could be even more affected as the impact of the war accumulates.

Rapeseed is one of the first major arable crops to be sown in Europe after summer harvesting and demand from Ukrainian growers so far was "relatively sluggish", Vilmorin's Chief Executive Franck Berger said.

"Some farmers are cautious about their capacity to carry out sowing," he said during a presentation of full-year sales.

Most of Vilmorin's sales in Ukraine come from spring-planted maize (corn) and sunflower crops, and it had little visibility over how the farming situation might evolve by then, he added.

For its 2021-2022 financial year that ended on June 30, Vilmorin recorded a 4% fall in combined sales for Ukraine and Russia to 134 million euros ($137.54 million), compared with a 7.5% rise in group annual sales to 1.587 billion euros.

Its sales in Ukraine dropped, marked by reduced planting by farmers and selling of existing stocks of sunflower seed by distributors, Vilmorin said.

In Russia, however, sales increased, partly due to late spring planting by some farmers that allowed Vilmorin to ship seed despite delays linked to the war and related sanctions.

Vilmorin has maintained commercial sales in Russia, citing its role in the global food supply chain, but has suspended a project to develop a local production facility.

($1 = 0.9743 euros) (Reporting by Gus Trompiz Editing by Marguerita Choy)