Russia and Ukraine have postponed the signing of a controversial gas deal, planned for Saturday, until next week.
Europeans are concerned about disruption to gas supplies
The reason given for putting off the signing was that some details needed to be finalised.
But the BBC's Helen Fawkes in Ukraine says it is likely that there were also political reasons for the delay.
The deal was reached earlier this month after Russia suspended gas supplies to Ukraine on New Year's Day, a move that affected several other countries.
Opposition to the deal in Ukraine led to political deadlock, and questions over whether the government had the authority to sign the agreement, which will double the price Kiev pays for gas.
Under the five-year deal, Ukraine will buy Russian and Central Asian gas for $95 per 1,000 cubic metres on average.
Since the agreement was reached, members of parliament have voted to sack the fuel and energy minister and the government, as well as passing a vote of "no confidence" in the head of the state gas company - although the authorities have said that these actions are illegal.
Our correspondent says this latest development will send a shiver across Europe, some parts of which have experienced a shortfall in gas supplies in recent weeks.
It is also feared that the delay could worsen the already strained relations between Russia and Ukraine.
Moscow is renegotiating gas deals with several of its neighbours.
The spat with Kiev led to disruption to European supplies, which run through Ukraine.
And this week, Italy said it was dipping into its gas reserves amid reports that Russia was holding back gas for its domestic markets because of cold weather at home.