* Gazprom says Ukraine owes it $3.3 billion for 2013-2014 gas deliveries
* Naftogas says may fail to pay on time due to client arrears
* Gazprom may require pre-payment by Kiev - report
By Katya Golubkova
MOSCOW, Feb 3 (Reuters) - Ukrainian energy firm Naftogas on Monday said non-payments by its clients mean it may fail to pay Gazprom on time for natural gas imports, a bill which the Russian monopoly says has reached $3.3 billion.
Moscow and Kiev have a history of troubled gas relations, often with regional implications, as most of Russia's exports to Europe are pumped through Soviet-built pipelines that traverse Ukraine.
Naftogas said in a statement that its largest customers owed it almost 27 billion hryvnias ($3.1 billion) as of Feb. 3, which may result in the company failing to pay Gazprom's bills on time.
Naftogas added that the debts of its largest clients had risen by almost 10 billion hryvnias over the last three months, and urged them to pay as soon as possible.
Gazprom said in a statement on Monday that Naftogas owed it $2.634 billion for 2013 deliveries and cited preliminary data for January that put Ukraine's supply at 2.45 billion cubic metres worth $658.4 million.
Quoting a Gazprom source, Russia's Vedomosti financial daily reported earlier on Monday that the company may ask Ukraine to pre-pay for gas imports, although no specific decision has been taken.
Gazprom did not comment on that matter in its statement.
The Russian gas giant has threatened in the past to invoke a clause in its supply contract that would allow it to demand pre-payment from Kiev, but has not done so.
Such a move could push Ukraine closer to default as President Viktor Yanukovich contends with protests against his rule while seeking to form a new government that would unlock further aid from Russia.
Cash-strapped Kiev late last year walked away from a trade deal with the European Union, instead agreeing to closer economic ties with Moscow in exchange for a $15 billion bailout ordered by President Vladimir Putin.
As a part of that deal, Russia agreed to cut the price it charges Kiev for gas by a third to $268.50 per 1,000 cubic metres.
Russia has so far disbursed $3 billion by buying Eurobonds issued by Ukraine. But a further tranche of $2 billion depends on the formation of a new government in Kiev after Prime Minister Mykola Azarov resigned last week.
Disputes over natural gas between Russia and Ukraine are important for Europe, which gets a quarter of its gas needs from Moscow. Gazprom exported 161.5 billion cubic metres of gas to Europe last year.
In previous years, Gazprom has sometimes cut supplies to Ukraine to push Kiev to pay for gas, resulting in lower flows to Europe during the winter heating season.