European and Russian officials will meet this week and next for emergency talks to try to prevent Moscow cutting off gas supply to Ukraine.
EU Energy Commissioner Guenther Oettinger will also meet Ukrainian and Slovakian ministers in Bratislava on Thursday to seek a deal on shipping gas to Ukraine via Slovakia in the event of a reduction of supplies.
But Russia has already warned that reversing the flow of gas through Slovakia is not legal.
"The solution we are proposing, which shall be put in place, this memorandum of understanding, is of course legally sound." EU Spokeswoman Sabine Berger said.
Since a new pro-Western government took over in Kiev in February, Moscow has nearly doubled the price it charges Ukraine for gas and it has also threatened to cut supplies.
Russia provides Europe with roughly one third of its gas imports, over half of which flowed via Ukraine last year. Russia also delivers around half of Ukraine's domestic gas needs.