* Donetsk, Luhansk regions harvested 3.5 mln T of grains in 2013
* Some fields in these regions too dangerous to harvest
* Donetsk, Luhansk have harvested 2.9 mln T so far in 2014
* Cargill says reports of a fire at one of its Donetsk silos (Adds detail, context on harvest in affected regions)
MOSCOW/KIEV, Aug 19 (Reuters) - Ukraine will lose 15 percent of its grain crop in two violence-hit regions in the country's east, a spokesman for Ukrainian Prime Minister Arseny Yatseniuk said.
The former Soviet republic, expected to be the world's second-largest grain exporter in the 2014/15 marketing year, has suffered major disruption since a pro-Russian uprising erupted in its eastern regions several months ago.
Earlier on Tuesday, Russia's Interfax news agency quoted Yatseniuk as saying Ukraine would lose 15 percent of its grain harvest due to the fighting and Russia's annexation of the Crimean peninsula earlier this year.
"This refers to 15 percent of the harvest in the regions where fighting is taking place," the spokesman for Yatseniuk said when asked to clarify the report.
Ukraine's Agriculture Ministry expects the 2014 grain harvest to exceed last year's record crop of 63 million tonnes. Analysts and traders have said Ukraine is likely to harvest around 60 million tonnes.
Last year, the Donetsk and Luhansk regions harvested 2.2 million tonnes and 1.3 million tonnes of grains, respectively, according to official statistics.
About 15 percent of their crop, based on last year's data, would be 500,000 tonnes, according to Reuters calculations, similar to an estimate of losses made in July by Ukrainian Agriculture Minister Ihor Shvaika.
So far this year, Donetsk and Luhansk have harvested 2 million tonnes and 0.9 million tonnes, according to the Agriculture Ministry.
"A certain percentage of fields in these regions continue to be dangerous for harvesting," the ministry said on its website. http://minagro.gov.ua
A group of armed individuals occupied a Cargill Inc sunflower-seed crushing plant in the Donetsk region in July after it was closed due to the increased tensions in the area.
Cargill is an agribusiness giant, one of the world's largest private companies.
"We have received reports that there was a fire at one of the grain silos at our facility in Donetsk which was attended by the local fire brigade. Unconfirmed reports suggest this may have been the result of a missile hit," a spokeswoman at Cargill said on Tuesday.
"No injuries and no damage to the rest of the facility have been reported. We are unable to make any further assessment of the facility at this time," the spokeswoman added.
Ukrainian farms harvested 34.7 million tonnes of grain in the whole country as of Aug. 15, including 24.1 million tonnes of wheat and 9.3 million tonnes of barley, the Agriculture Ministry said. (Reporting by Polina Devitt in Moscow, Pavel Polityuk in Kiev and Sarah McFarlane in London; Additional reporting by Maria Tsvetkova in Donetsk; Editing by Michael Urquhart and Alexander Winning)