UPDATE 1-Germany warns of Russian gas shortfalls

* E.ON Ruhrgas sees shipments via Ukraine massively reduced

* No supply guarantees if delivery shortfalls last

* Shares in E.ON and BASF fall on the news, but still higher

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FRANKFURT, Jan 6 (Reuters) - German energy firms warned on Tuesday there could be gas shortages in Europe's biggest economy if the dispute between Moscow and Kiev lasted much longer and sub-zero temperatures endured.

"Even our possibilities will reach their limits if these drastic cuts in shipments last and if temperatures continue to stay at very low levels," E.ON Ruhrgas Chief Executive Bernhard Reutersberg said.

Germany, which receives more than 40 percent of its gas from Russia, has been hit by a cold spell, with temperatures in some parts of the country reaching their lowest levels in almost 30 years on Monday night.

E.ON Ruhrgas (EONGn.DE) and BASF's Wingas (BASF.DE) said they were seeing reductions to gas flows on Ukraine's transit routes to Germany and were turning to gas reserves, alternative shipping routes and other supplier countries.