Russian energy minister Alexander Novak has warned that EU states which re-export gas to Ukraine will face cut-offs, with Hungary already stopping its reverse flow.

Novak spoke in German daily Handelsblatt on Friday (26 September) morning ahead of talks in Berlin later the same day between the European Commission and Russian and Ukrainian energy officials.

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"The contracts signed [between Russia and EU clients] do not have any provisions for re-exports … We hope that our European partners respect the past agreements. That is the only way to guarantee uninterrupted supplies”, he said.

Hungary, Poland, and Slovakia earlier this year began shipping Russian gas to Ukraine in order to help it cope in winter after Russia stopped supplying Ukraine in a price dispute which is bogged down in legal arbitration.

Earlier this month, Russian gas flows to Poland temporarily dropped by 45 percent and to Slovakia by 10 percent in what was blamed on technical reasons.

For its part, Hungarian gas distributor FGSZ on Thursday said it will no longer supply Ukraine “until further notice” also due to “technical reasons” linked to increased demand at home.

But Ukraine transit firm Naftogaz said in a statement the decision is political and comes after the CEO of Russian gas firm Gazprom, Alexei Miller, met with Hungarian leader Viktor Orban on Monday.

“Neither EU countries nor Ukraine should be put under political pressure through energy blackmail”, it said in a note to press. “Naftogaz is committed to being a reliable partner and vital transit hub for Russian gas to the EU. In return, Naftogaz expects its European partners and neighbours to respect their contractual obligations”.

Ukrainian PM Arseny Yatseniuk told Reuters: “They want us to freeze. This is the aim and this is another trump card in Russian hands".

The commission hopes the Berlin meeting will see Russia and Ukraine agree to an interim gas price to restart supplies during the cold weather.

But two recent EU-brokered deals, on a ceasefire in east Ukraine and on EU-Ukraine free trade, have attracted criticism.

The ceasefire, agreed on 5 September, has seen Ukraine pass laws to let pro-Russia rebels stay in place in Donetsk and Luhansk and to create a buffer zone between the two forces.

Ukrainian president Petro Poroshenko told press in Kiev on Thursday that the “most dangerous part of the war is over” and unveiled a reform plan designed to prepare Ukraine to apply for EU membership in 2020.

EU countries’ ambassadors will next week in Brussels discuss whether Russia has done enough to merit rolling back EU sanctions.

But speaking to the Washington Post in an interview on the margins of the UN assembly in New York on Thursday, Lithuanian leader Dalia Grybauskaite, said: “The Europeans and other countries asked Poroshenko to agree to conditions that were unacceptable because [they amounted to a partition] of Ukrainian territory”.

She added: “If we will be too soft with our sanctions or adapt sanctions but not implement them, I think he [Russian leader Vladimir Putin] will go further trying to unite east Ukraine with south Ukraine and Crimea”.

The EU-brokered trade deal has also seen Ukraine promise to freeze implementation of a landmark free trade pact until 2016.

But Russian leader Vladimir Putin has said in a letter to the EU commission, seen by Reuters, that he will impose trade sanctions on Ukraine if it starts to make any pro-EU economic reforms in the next 15 months.

He added that the substance of the trade treaty must be modified to meet Russian demands.

“We still believe that only systemic adjustments of the association agreement, which take into account the full range of risks to Russian-Ukrainian economic ties and to the whole Russian economy arising from implementation of the agreement, will allow [us] to retain existing trade and economic considerations between the Russian Federation and Ukraine”, he said.