(QHA) - On Oct. 29 Russian Gazprom hit Ukraine with just short of a $1 billion gas bill in retribution for Kyiv’s decision to turn its back on the Russian-led Customs Union and throw in its lot with the EU with the signing of a free trade and association pact at a November EU summit in Lithuania. It has also rekindled talk of a gas war between the one-time allies, reports Business New Europe. "We're very concerned about the situation with Ukraine's payment for Russian gas supplies. We signed an additional agreement to the contract of gas deliveries worth $882m in August, which set a payment deadline of October 1," Gazprom CEO Alexei Miller said on October 29. "But October is drawing to an end, yet the gas bill remains unpaid, so the situation is very serious, taking into account the fact that the contract envisions the prepayment option in case of violation of terms."- he added. Ukraine's state energy firm Naftogaz declined to comment on Gazprom's remarks, but Ukrainian Prime Minister Nicolay Azarov said the government was "monitoring the issue". "This, primarily, is a question concerning two companies, and they have to sort things out," Azarov's was quoted as saying.