MOSCOW — As Russia warned Ukraine on Tuesday that it could run out of natural gas within two days because of a dispute over payments, Britain said it was sending military trainers to aid Ukrainian forces while European diplomats labored to patch up a flagging peace agreement in eastern Ukraine.  The warning by Aleksei B. Miller, the chief executive of Gazprom, the state-controlled Russian energy behemoth, illustrated how the hostilities between Russia and Ukraine are hardly limited to the battlefields in eastern Ukraine, where a cease-fire agreed to nearly two weeks ago has still not fully taken hold.  Russia has long used its muscle as the region’s major energy supplier to wield political and economic influence, particularly in disputes with Ukraine and other former Soviet republics. A bitter feud over gas payments has been a subplot of the wider political dispute between Moscow and Kiev over the past year.  As part of that dispute, Russia last year cut off supplies and ended a longstanding practice of selling gas to Ukraine on credit, and instead demanded prepayment.  “Ukraine has not made prepayment for gas on time,” Mr. Miller said at a news conference in Russia, local news agencies reported. He added that the time needed for Kiev to make a payment “will result in a total end to supplies of Russian gas to Ukraine in just two days, which poses serious risks for gas transit to Europe.”  However, Ukraine says it has already paid for all the gas it requested for this year, and for an additional 287 million cubic meters not yet ordered. Kiev is now accusing Russia of violating an agreement reached in October, under which Ukraine paid $3.1 billion in past gas bills and Gazprom resumed supplies on a prepaid basis. That agreement was expected to keep Ukraine fully supplied with gas through the winter.  The dispute seems to hinge at least in part on the gas that Russia has delivered to the breakaway regions in eastern Ukraine, which it says counts toward the total Kiev bought in advance. Earlier this month, Gazprom said it would supply natural gas directly to the regions, which are largely controlled by separatists, because it said the Ukrainian government had shut off supplies. Gazprom said that it would charge Ukraine for that gas, and that the amount of gas supplied to the east would be deducted from Ukraine’s prepaid allotment.  Kommersant, the Russian business newspaper, reported that Gazprom had slowed deliveries over pipelines crossing Ukrainian-controlled parts of the border, while opening the spigots to two pipelines leading directly to rebel-held territory.  The gas diversion highlights a broader Russian strategy in eastern Ukraine of assuring its political and military control over the breakaway enclave while avoiding the economic burden of caring for the population, estimated at about three million people. In the truce talks, President Vladimir V. Putin held out for measures to ensure that the Ukrainian authorities would pay public-sector wages and pensions and reopen banks.  Gazprom, Kommersant reported, rejected the Ukrainian government’s argument that gas delivered to rebel territory could not be counted against its prepaid volumes, at least because Naftogaz, the state energy company, could not send meter readers to the two border crossing points, Prokhorov and Platovo, to confirm deliveries.  As the gas Ukraine had already paid for flowed into their territories, separatist leaders went on local television to thank Russia and Mr. Putin. “We thank the Russian Federation and Vladimir Vladimirovich, as Russia is again extending its hand to help, giving warmth,” said the leader of the Luhansk People’s Republic, Igor Plotnitsky. “Thank you, Russia.”  Mindful of how Russia has used gas as a political weapon, Ukraine has taken strong steps in recent months to reduce its dependence on it. Ukraine and Slovakia reached a deal for reverse piping of gas already purchased in Europe, and a separate deal to buy gas from Norway.  And despite Mr. Miller’s comments, there was no reason to believe that Europe would find itself short of gas as a result of the dispute. Europe, too, has reduced its dependence on Russian gas by engaging other suppliers, and in recent months has built up reserves in anticipation of potential difficulties with Russia.  Russia, while controlling the supply, is in turn dependent on Ukraine to allow gas through its pipelines to other customers in Europe. Painfully aware of that reliance, Russia has sought to cut Ukraine out by building a new pipeline under the Black Sea and through Turkey.  The continuing gas dispute demonstrated the extent to which Ukraine would still be at Russia’s mercy even without the war against Russian-backed separatists. Fighting, however, has continued despite the cease-fire brokered this month in Minsk, Belarus, and on Tuesday, the foreign ministers of Russia, Ukraine, France and Germany met in Paris in a bid to get it back on track. It did not appear that they had much success.  In a statement after the meeting, the French foreign minister, Laurent Fabius, said the four nations remained committed to the Minsk accord and were demanding that all sides observe the truce without exception.  In an appearance before Parliament on Tuesday, Prime Minister David Cameron of Britain spoke out strongly against Russian aggression in Ukraine, pledging to send up to 75 military advisers and raising the possibility of cutting Russia off from the global Swift banking system, as was done to Iran over its nuclear program.  “If Russia is going to leave the rules-based system of the 21st century,” Mr. Cameron said, “then it has to start thinking about whether it’s going to be in the 21st-century system when it comes to investment, when it comes to banking, when it comes to clearinghouses.”  Mr. Fabius said the foreign ministers were concerned about the coastal city of Mariupol. Ukraine and its allies have long feared that the separatists would make a push for Mariupol as part of a bid to establish a land connection to Crimea, which Russia invaded and annexed last March.  The four foreign ministers also called for unfettered access throughout the conflict zone for observers from the Organization for Security and Cooperation in Europe, which is charged with monitoring the cease-fire deal.  Aurelien Breeden contributed reporting from Paris.  A version of this article appears in print on February 25, 2015, on page A7 of the New York edition with the headline: Russia Heightens Dispute With Ukraine Over Natural Gas. Order Reprints| Today's Paper|Subscribe
The head of Gazprom said that Ukraine would use up its prepaid allotment in days and would not be able to pay in time to keep the supply going.