The EU said it hoped for a "positive outcome" Monday as the Russian and Ukrainian energy ministers held talks in Brussels aimed at resolving gas supply dispute threatening deliveries to Europe.
Russian state-owned gas giant Gazprom threatened last week to cut deliveries to Ukraine and divert supplies instead to eastern parts of the country controlled by pro-Kremlin rebels.
The European Union receives about a third of its gas from Russia, with half that amount transiting via Ukraine pipelines.
European Commission Vice-President for Energy Union Maros Sefcovic opened the talks with energy ministers Alexander Novak of Russia and Volodymyr Demchyshyn of Ukraine at 1530 GMT, an hour and a half late.
"Hope for positive outcome," Sefcovic said in a tweet earlier.
Tensions rose last month when Gazprom began supplying gas directly to separatist-held areas in eastern Ukraine and demanded Kiev pay for it.
Rebel leaders in east Ukraine said that Kiev had suddenly ceased gas supplies and asked for access to gas from Russia, which is accused of backing the separatists with troops, weapons and other types of assistance.
Russia denies that it supports the rebels militarily.
Naftogaz, Ukraine's national gas company, confirmed the cut, saying it was due to pipeline damage caused by fighting.
The Ukrainian and Russian gas firms were to be represented in Monday's talks, but Gazprom CEO Alexei Miller was not due to attend.
Russian President Vladimir Putin has said that Kiev's move to cut gas supply smacked of "genocide" and targeted four million people living in the area.
The talks are the latest in a series that Brussels has hosted over the past year to resolve the gas crisis facing Ukraine.
The European Commission said it expected both sides to continue to respect a "winter package" deal mediated by the EU late last year which guaranteed Russian gas supplies to Ukraine through the end of March.
"The main aim is to discuss the implementation of the winter package. That is the starting point," Commission spokeswoman Anna-Kaisa Itkoken told a press briefing Monday.
"The Commission is pleased to see that both sides will come to Brussels today to try to find a solution," she added before the talks began.
But she played down the chances of a longer-term settlement for the summer, saying it was only the immediate dispute that was going to be discussed.
In earlier disputes in 2006 and 2009, Gazprom halted deliveries to Kiev, causing huge disruption to European gas supplies at the height of winter.
A written statement will be released following the trilateral talks, but no media briefing is scheduled, commission officials said.