"The Ukrainian government invited a delegation from Gazprom for talks ... We have warned them that supplies could be reduced from September 1," Chief Engineer Vladimir Podmarkov said.
He said Ukraine had not taken sufficient steps to ensure payments were made after an agreement between the two former Soviet republics led to supplies being restored in April.
The April agreement followed sharp supply cuts and accusations by Gazprom that gas intended for Western European customers was being siphoned from pipelines crossing Ukrainian territory.
Ukraine, heavily dependent on Russia for oil and gas supplies but unable to pay, owes Gazprom more than two trillion rubles ($950 million) for past shipments, Podmarkov said.
The Gazprom delegation, headed by company chairman Rem Vyakhirev, will discuss ways of paying off the debt. Podmarkov said the latest threat to cut supplies would be carried out in the absence of a settlement, but he could not predict how much supplies would be reduced by.
In Kiev, a spokesman for the state committee on oil and gas acknowledged that Ukraine was failing to meet payment obligations to Gazprom.
"Ukraine is only paying 10 to 20 percent of its obligations, and Gazprom has demanded that we immediately pay off our gas debt," Stepan Yaloveha said.
Earlier this year, Gazprom threatened Kiev with a complete cut-off, raising the prospect of severe disruptions to western European consumers. More than 90 percent of Russian gas exports to Western Europe cross Ukraine.
But Gazprom's big clients in Germany, Italy, France and other countries noticed no significant impact from the dispute as reductions in pipeline pressure were brief and stocks ample.
Still, "If we have another early cold winter, then we could have a repeat of similar problems," Yaloveha said.
He said Kiev should take steps to collect money owed by domestic enterprises and repay its debts to Gazprom. Ukrainian enterprises owed the state committee 14.6 trillion karbovanets ($320 million at commercial rates) for gas.
"The price we charge enterprises for gas should reflect its real price," he said, referring to big government subsidies.
"And the committee thinks Ukraine must start constructing housing and supplying goods to Russia."
Russia and Ukraine have started working out mechanisms for Ukraine to pay off some of its debt with food products and services such as housing construction.
Gazprom's agreement with Ukraine in April also called for Kiev to clear some of its debt with shares in privatized gas storage and transport facilities.
But Ukrainian authorities have not yet approved Gazprom's participation in privatization.
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