Naftogaz Ukrainy has transferred $40 million to Gazprom as prepayment for natural gas supplies in May, a Naftogaz Ukrainy spokesman told Interfax.

"Gazprom was paid $40 million today," the spokesman said.

Naftogaz doubled gas imports from Russia since the beginning of May, the Ukrainian gas pipeline operator Ukrtransgaz said. If in the last ten days of April Naftogaz was receiving on average 11 million cubic meters of gas a day, it imported 18-20 mcm a day in the first three days of May.

Interfax calculates that as of Tuesday morning, Naftogaz had just under 100 mcm of prepaid gas left to receive from Gazprom.

In January-April 2015, Ukraine imported 2.451 bcm of gas from Russia, including 295 million cubic meters in April.