* Russia says bill is for shortfall in volumes of gas imports
* Ukraine seeks to wean itself off costly Russian gas
* Dampens mood after Ukraine's landmark Shell shale deal
By Pavel Polityuk and Richard Balmforth
KIEV, Jan 26 (Reuters) - Russia has slapped a $7 billion bill on Ukraine for imports of gas it says Kiev was committed to buy last year but never took, a Ukrainian energy industry source said on Saturday.
The move by Moscow comes just days after the former Soviet republic signed a $10 billion deal with Royal Dutch Shell for shale gas exploration aimed at reducing Ukraine's reliance on costly Russian gas imports.
It also handed a fresh concern to President Viktor Yanukovich's government as it prepared to meet an International Monetary Fund mission next Tuesday for talks on a possible new $15 billion loan programme to help Kiev meet foreign debt repayments this year.
"We have indeed received a bill for $7 billion," the industry source told Reuters.
But the source said the Kiev government challenged the calculation by Russian gas giant Gazprom and saw a political tinge linked to Thursday's shale deal signed at the World Economic Forum in Davos, Switzerland.
Past tensions over energy between the former Soviet allies erupted into open dispute in 2006 and 2009 causing disruption to Russian gas flows via Ukraine to the European Union.
There was no immediate comment from Russian gas giant Gazprom in Moscow or from Ukraine's state gas company Naftogaz. Ukraine's government had no immediate comment either.
"In our opinion, we don't owe anything to anyone. All the gas which we should have bought (last year) according to the contract, we bought and have paid for," the energy industry source said.
"It looks as if everything is linked to the signing of the agreement with Shell in Davos," he said.
Relations between Gazprom and Naftogaz have been especially fraught since the 2009 signing of a 10-year supply agreement brokered by then prime minister Yulia Tymoshenko.
She is now serving a seven-year jail sentence for abuse-of-office linked to the contract.
Under the deal, Ukraine currently pays about $430 per 1,000 cubic metres for Russian gas - above market price - and is also committed to importing fixed annual volumes of gas under a 'take-or-pay' clause.
The present Kiev government of Mykola Azarov says the price is exorbitant but has so far failed to persuade Russia to lower it.
Moscow has also been unsympathetic to Ukraine's calls for agreed volumes of imported gas to be reduced.
Annual contract volumes can be changed by mutual agreement no later than six months before the start of a calendar year.
This change should not exceed 20 percent of the original contract volume and Ukraine is obliged to take or pay for a minimum of 80 percent of the annual contracted volume, according to a copy of the contract published by a Ukrainian news site in 2009.
The source said Gazprom had calculated that Ukraine's Naftogaz had been committed to importing 42 billion cubic metres of gas in 2012 under the supply deal.
Ukraine in fact imported 33 bcm last year, the minimum allowed under the contract, he said.
Though most of this was by Naftogaz, a relatively small portion of it had been bought by a private Ukrainian gas trader which may have led to Gazprom inflating its calculations of a shortfall, he said.
Ukraine did not regard the (Gazprom) calculation as reliable, the source added.
The Azarov government, reshuffled following an October election, has been hoping that its efforts to seek alternative supplies, including tapping unconventional sources such as shale gas, would strengthen its hand in price talks with Moscow.
Under the 50-year production-sharing agreement signed in Davos, Shell will develop the vast Yuzivska field in the east of Ukraine to tap into national reserves estimated at 42 trillion cubic feet (1.2 trillion cubic metres) by the U.S. Energy Information Administration.
Ukrainian officials also see prospects of more shale exploration with U.S. energy giant Chevron and offshore exploration in the Black Sea with an ExxonMobil-led consortium.
Editing by Jason Neely