EU leaders meet Tuesday with Ukrainian President Volodymyr Zelensky, who has fiercely opposed a new gas pipeline that bypasses his country and increases Europe's energy reliance on Russia.
European Council leader Charles Michel and European Commission leader Ursula von der Leyen are heading to Ukraine as Europe faces a gas crisis after a surge in prices and drop in reserves.
Ukraine's years-long war with Russian-backed separatists and reform efforts will also be on the agenda, but the gas crisis is likely to be at the forefront of talks, reported AFP.
"This summit will take place in a very tense atmosphere," said Leonid Litra, an analyst at New Europe Centre, a Kiev-based think tank.
Ukraine -- in conflict with Russia since Moscow's 2014 annexation of Crimea -- wants to ensure it will remain a key transit country and Zelensky has spoken out against Nord Stream 2, a Baltic Sea pipeline set to double natural gas supplies from Russia to Germany.
The pipeline, which is still awaiting approval from a German regulator, diverts supplies from an existing route through Ukraine and is expected to deprive Europe's ally of an estimated one billion euros ($1.1 billion) annually in transit fees from Russia.
On Tuesday, Michel and von der Leyen will try to reassure Zelensky, a European official said on condition of anonymity.
They will insist on the European commitment to "of course" guarantee "the role of Ukraine as a gas transit country", the official told reporters.
No major advances are expected, however.
Ahead of the summit, the EU on Monday imposed sanctions on eight officials accused of targeting opponents of Russia's seizure of Crimea.
- Ukraine feels 'ignored' -
Critics have accused Moscow of intentionally limiting gas supplies to Europe and driving up prices in an effort to hasten the launch of Nord Stream 2, a claim Russia has denied.
President Vladimir Putin stressed last week that his priority was not to put Ukraine "in a difficult position", but to be an "absolutely reliable partner" of Europe in the energy market.
At the same time the Kremlin chief said that Ukrainian gas pipelines had not been repaired "for decades" and ramping up supplies via the post-Soviet country could lead to "negative consequences."
"Something can burst there at any moment," Putin said.
Moscow has not booked additional gas transit capacity via Ukraine to Europe for October, raising concerns.
Russia denies any pressure, saying it needs to fill its own reserves for the winter before sending supplies on to Europe.
Since 2014, Ukraine's army has been fighting pro-Russian separatists in the country's east in a conflict that has claimed more than 13,000 lives.
Kiev accuses Europe of timidity when it comes to Russia, whether on gas, military cooperation or Ukraine's prospects for integration into NATO and the European Union.
For its part, Europe has repeatedly called on Kiev to commit greater efforts to combating graft and reforming Ukraine's notoriously corrupt judicial system.
"We are going to encourage our Ukrainian friends to go a little faster," the European official said.
The summit will also be an opportunity for Europeans to "reaffirm their commitment to strengthen Ukraine's political association and economic integration with the EU", the official added.
"There is tension and grievances on both sides", and Ukraine feels "ignored," said Litra of the New Europe Centre think tank.
