Ukraine has previously ruled out extending a contract to transit Russian gas, via Ukraine, to Europe, which is due to expire at the end of the year. Slovakia, one of the recipients of the gas, has been racing to prolong the deal.

"We would not prolong the transit of Russian gas. We will not give the possibility of additional billions to be earned on our blood, on the lives of our citizens," Zelenskiy told a news conference during a European Union summit in Brussels.

But he added: "If the country is ready to give us the gas, but not to pay the money back to Russia until the end of the war, then it's a possible potentiality. We can think about it."

The European Commission has said it is ready for the contract to expire, and all countries receiving Russian fuel via the Ukraine route have access to alternative supplies.

Zelenskiy lambasted Slovak Prime Minister Robert Fico, who has warned of the economic hit his country will face if it loses cheap gas from Russia.

"To be honest, during war, it's a bit shameful to talk about money, because we are losing people," Zelenskiy said.

Zelenskiy said he told Fico on Thursday that Ukraine would be open to carrying another country's gas through its infrastructure to reach Europe - but it would need assurances that this was not merely re-labelled Russian fuel.

"We have to know that we will only transit gas if it's not coming from Russia," Zelenskiy said.